diff --git a/configure.ac b/configure.ac
index d113e888d..fcabd1fb3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -87,8 +87,7 @@ AS_IF([test "x$enable_compile_warnings" != "xno"],
 AS_IF([test "x$enable_compile_warnings" != "xno"],
   [ax_enable_compile_warnings=yes
    AX_COMPILER_FLAGS_CFLAGS([WARNING_CFLAGS], [], [ dnl
-       -pedantic dnl
-       -Wold-style-cast])
+       -pedantic dnl])
    AC_SUBST([WARNING_CFLAGS])])
 
 # Check for system planarity suite
diff --git a/extern/bliss-0.73/bliss_C.cc b/extern/bliss-0.73/bliss_C.cc
index 283bbbf5e..7652d5354 100644
--- a/extern/bliss-0.73/bliss_C.cc
+++ b/extern/bliss-0.73/bliss_C.cc
@@ -29,6 +29,11 @@ extern "C" {
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wold-style-cast"
 #pragma clang diagnostic ignored "-Wsign-compare"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wold-style-cast"
+#pragma GCC diagnostic ignored "-Wsign-compare"
+#pragma GCC diagnostic ignored "-Wmissing-declarations"
 #endif
 
 struct bliss_digraphs_graph_struct {
@@ -238,4 +243,6 @@ void bliss_digraphs_free_blissstats(BlissStats *stats)
 
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
diff --git a/extern/bliss-0.73/defs.cc b/extern/bliss-0.73/defs.cc
index 86ccc63f6..d14315018 100644
--- a/extern/bliss-0.73/defs.cc
+++ b/extern/bliss-0.73/defs.cc
@@ -25,6 +25,11 @@
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wformat-nonliteral"
 #pragma clang diagnostic ignored "-Wmissing-noreturn"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wformat-nonliteral"
+#pragma GCC diagnostic ignored "-Wmissing-noreturn"
+#pragma GCC diagnostic ignored "-Wsuggest-attribute=format"
 #endif
 namespace bliss_digraphs {
 
@@ -42,4 +47,6 @@ namespace bliss_digraphs {
 
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
diff --git a/extern/bliss-0.73/graph.cc b/extern/bliss-0.73/graph.cc
index f1f12724c..56f6ef503 100755
--- a/extern/bliss-0.73/graph.cc
+++ b/extern/bliss-0.73/graph.cc
@@ -33,6 +33,9 @@
 #if defined(__clang__)
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wold-style-cast"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wold-style-cast"
 #endif
 
 namespace bliss_digraphs {
diff --git a/extern/bliss-0.73/timer.cc b/extern/bliss-0.73/timer.cc
index d69366fdd..a3cd06e91 100644
--- a/extern/bliss-0.73/timer.cc
+++ b/extern/bliss-0.73/timer.cc
@@ -24,6 +24,10 @@
 #if defined(__clang__)
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wold-style-cast"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wold-style-cast"
+#pragma GCC diagnostic ignored "-Wuninitialized"
 #endif
 
 namespace bliss_digraphs {
@@ -60,4 +64,6 @@ double Timer::get_duration()
 } // namespace bliss_digraphs
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
diff --git a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphDFSUtils.c b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphDFSUtils.c
index 9ac87b027..91e45d90d 100644
--- a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphDFSUtils.c
+++ b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphDFSUtils.c
@@ -31,6 +31,10 @@ extern void _ClearVertexVisitedFlags(graphP theGraph, int);
 #pragma clang diagnostic ignored "-Wstrict-prototypes"
 #pragma clang diagnostic ignored "-Wmissing-prototypes"
 #pragma clang diagnostic ignored "-Wdeprecated-non-prototype"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
 #endif
 
 int  gp_CreateDFSTree(graphP theGraph)
@@ -459,4 +463,6 @@ printf("LeastAncestor in %.3lf seconds.\n", platform_GetDuration(start,end));
 }
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
diff --git a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphDrawPlanar.c b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphDrawPlanar.c
index 1b107b04a..a63df776e 100644
--- a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphDrawPlanar.c
+++ b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphDrawPlanar.c
@@ -9,6 +9,10 @@ See the LICENSE.TXT file for licensing information.
 #pragma clang diagnostic ignored "-Wstrict-prototypes"
 #pragma clang diagnostic ignored "-Wmissing-prototypes"
 #pragma clang diagnostic ignored "-Wdeprecated-non-prototype"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
 #endif
 
 #include "graphDrawPlanar.h"
@@ -1047,4 +1051,6 @@ int v, e, eTwin, EsizeOccupied, epos, eposIndex;
 }
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
diff --git a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphDrawPlanar_Extensions.c b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphDrawPlanar_Extensions.c
index c8b796689..3cb27b1b2 100644
--- a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphDrawPlanar_Extensions.c
+++ b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphDrawPlanar_Extensions.c
@@ -3,20 +3,22 @@ Copyright (c) 1997-2020, John M. Boyer
 All rights reserved.
 See the LICENSE.TXT file for licensing information.
 */
-
-#if defined(__clang__)
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wstrict-prototypes"
-#pragma clang diagnostic ignored "-Wmissing-prototypes"
-#pragma clang diagnostic ignored "-Wdeprecated-non-prototype"
-#endif
+
+#if defined(__clang__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wstrict-prototypes"
+#pragma clang diagnostic ignored "-Wmissing-prototypes"
+#pragma clang diagnostic ignored "-Wdeprecated-non-prototype"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
+#endif
 
 #include <stdlib.h>
 
 #include "graphDrawPlanar.private.h"
 #include "graphDrawPlanar.h"
-
-
 
 extern void _ClearVertexVisitedFlags(graphP theGraph, int);
 
@@ -59,7 +61,7 @@ void _DrawPlanar_FreeContext(void *);
 /****************************************************************************
  * DRAWPLANAR_ID - the variable used to hold the integer identifier for this
  * extension, enabling this feature's extension context to be distinguished
- * from other features' extension contexts that may be attached to a graph.
+ * from other features' extension contexts that may be attached to a graph.
  ****************************************************************************/
 
 int DRAWPLANAR_ID = 0;
@@ -687,6 +689,8 @@ int  _DrawPlanar_WritePostprocess(graphP theGraph, void **pExtraData, long *pExt
 
     return NOTOK;
 }
-#if defined(__clang__)
-#pragma clang diagnostic pop
-#endif
+#if defined(__clang__)
+#pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
+#endif
diff --git a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphEmbed.c b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphEmbed.c
index a0d2ec7f1..81f3ec48d 100644
--- a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphEmbed.c
+++ b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphEmbed.c
@@ -13,6 +13,10 @@ See the LICENSE.TXT file for licensing information.
 #pragma clang diagnostic ignored "-Wstrict-prototypes"
 #pragma clang diagnostic ignored "-Wmissing-prototypes"
 #pragma clang diagnostic ignored "-Wdeprecated-non-prototype"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
 #endif
 
 /* Imported functions */
@@ -1369,4 +1373,6 @@ int  e_u, e_v, e_ulink, e_vlink;
 
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
diff --git a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphExtensions.c b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphExtensions.c
index 1d46eba11..199603bd8 100644
--- a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphExtensions.c
+++ b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphExtensions.c
@@ -13,6 +13,13 @@ See the LICENSE.TXT file for licensing information.
 #include "graphExtensions.h"
 #include "graphFunctionTable.h"
 
+#ifdef __GNUC__
+    #ifndef __clang__
+    #pragma GCC diagnostic push
+    #pragma GCC diagnostic ignored "-Wshadow"
+    #endif
+#endif
+
 /* Imported functions */
 
 extern void _InitFunctionTable(graphP theGraph);
@@ -30,7 +37,7 @@ graphExtensionP _FindNearestOverload(graphP theGraph, graphExtensionP target, in
  * An ID identifies an extension, which may be added to multiple
  * graphs.  It is used in lieu of identifying extensions by a string
  * name, which is noticeably expensive when a frequently called
- * overload function seeks the extension context for a graph.
+ * overload function seeks the extension context for a graph.
  ********************************************************************/
 
 static int moduleIDGenerator = 0;
@@ -544,3 +551,9 @@ void _FreeExtension(graphExtensionP extension)
     }
     free(extension);
 }
+
+#ifdef __GNUC__
+    #ifndef __clang__
+    #pragma GCC diagnostic pop
+    #endif
+#endif
\ No newline at end of file
diff --git a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphFunctionTable.h b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphFunctionTable.h
index 46fc6cb31..24cbef0b4 100644
--- a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphFunctionTable.h
+++ b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphFunctionTable.h
@@ -21,6 +21,10 @@ extern "C" {
 #pragma clang diagnostic ignored "-Wstrict-prototypes"
 #pragma clang diagnostic ignored "-Wmissing-prototypes"
 #pragma clang diagnostic ignored "-Wdeprecated-non-prototype"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
 #endif
 
 typedef struct
@@ -65,6 +69,8 @@ typedef graphFunctionTable * graphFunctionTableP;
 
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
 
 #ifdef __cplusplus
diff --git a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphIO.c b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphIO.c
index f268ab65b..9b805ed3d 100644
--- a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphIO.c
+++ b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphIO.c
@@ -10,6 +10,13 @@ See the LICENSE.TXT file for licensing information.
 #pragma clang diagnostic ignored "-Wdeprecated-non-prototype"
 #pragma clang diagnostic ignored "-Wincompatible-pointer-types-discards-qualifiers"
 #pragma clang diagnostic ignored "-Wformat-nonliteral"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
+#pragma GCC diagnostic ignored "-Wdiscarded-qualifiers"
+#pragma GCC diagnostic ignored "-Wformat-nonliteral"
+#pragma GCC diagnostic ignored "-Wunused-result"
 #endif
 
 #include <stdlib.h>
@@ -762,4 +769,6 @@ char *_MakeLogStr5(char *format, int one, int two, int three, int four, int five
 }
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
diff --git a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphIsolator.c b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphIsolator.c
index e85317c6f..5f5b0560a 100644
--- a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphIsolator.c
+++ b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphIsolator.c
@@ -9,6 +9,10 @@ See the LICENSE.TXT file for licensing information.
 #pragma clang diagnostic ignored "-Wstrict-prototypes"
 #pragma clang diagnostic ignored "-Wmissing-prototypes"
 #pragma clang diagnostic ignored "-Wdeprecated-non-prototype"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
 #endif
 #define GRAPHISOLATOR_C
 
@@ -831,4 +835,6 @@ int  _DeleteUnmarkedVerticesAndEdges(graphP theGraph)
 }
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
diff --git a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK23Search.c b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK23Search.c
index 320226879..dd3ecd0d3 100644
--- a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK23Search.c
+++ b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK23Search.c
@@ -9,6 +9,10 @@ See the LICENSE.TXT file for licensing information.
 #pragma clang diagnostic ignored "-Wstrict-prototypes"
 #pragma clang diagnostic ignored "-Wmissing-prototypes"
 #pragma clang diagnostic ignored "-Wdeprecated-non-prototype"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
 #endif
 #include "graph.h"
 
@@ -271,4 +275,6 @@ int u, d, XorY;
 }
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
diff --git a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK23Search_Extensions.c b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK23Search_Extensions.c
index 9384b1bce..5a7c246b6 100644
--- a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK23Search_Extensions.c
+++ b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK23Search_Extensions.c
@@ -9,6 +9,10 @@ See the LICENSE.TXT file for licensing information.
 #pragma clang diagnostic ignored "-Wstrict-prototypes"
 #pragma clang diagnostic ignored "-Wmissing-prototypes"
 #pragma clang diagnostic ignored "-Wdeprecated-non-prototype"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
 #endif
 
 #include <stdlib.h>
@@ -255,4 +259,6 @@ int  _K23Search_CheckObstructionIntegrity(graphP theGraph, graphP origGraph)
 }
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
diff --git a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK33Search.c b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK33Search.c
index 78c2f64cc..cd8133c6e 100644
--- a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK33Search.c
+++ b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK33Search.c
@@ -9,6 +9,10 @@ See the LICENSE.TXT file for licensing information.
 #pragma clang diagnostic ignored "-Wstrict-prototypes"
 #pragma clang diagnostic ignored "-Wmissing-prototypes"
 #pragma clang diagnostic ignored "-Wdeprecated-non-prototype"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
 #endif
 
 #include "graphK33Search.h"
@@ -2055,4 +2059,6 @@ int u_min, u_max, d, u_d;
 }
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
diff --git a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK33Search_Extensions.c b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK33Search_Extensions.c
index fee63863e..60d14ed73 100644
--- a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK33Search_Extensions.c
+++ b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK33Search_Extensions.c
@@ -9,6 +9,10 @@ See the LICENSE.TXT file for licensing information.
 #pragma clang diagnostic ignored "-Wstrict-prototypes"
 #pragma clang diagnostic ignored "-Wmissing-prototypes"
 #pragma clang diagnostic ignored "-Wdeprecated-non-prototype"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
 #endif
 #include <stdlib.h>
 
diff --git a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK4Search.c b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK4Search.c
index fba6729f9..5179df39b 100644
--- a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK4Search.c
+++ b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK4Search.c
@@ -9,6 +9,10 @@ See the LICENSE.TXT file for licensing information.
 #pragma clang diagnostic ignored "-Wstrict-prototypes"
 #pragma clang diagnostic ignored "-Wmissing-prototypes"
 #pragma clang diagnostic ignored "-Wdeprecated-non-prototype"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
 #endif
 #include "graphK4Search.h"
 #include "graphK4Search.private.h"
@@ -1481,4 +1485,6 @@ int  _K4_RestoreAndOrientReducedPaths(graphP theGraph, K4SearchContext *context)
 }
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
diff --git a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK4Search_Extensions.c b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK4Search_Extensions.c
index 046fa63f6..9338beb63 100644
--- a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK4Search_Extensions.c
+++ b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphK4Search_Extensions.c
@@ -9,6 +9,10 @@ See the LICENSE.TXT file for licensing information.
 #pragma clang diagnostic ignored "-Wstrict-prototypes"
 #pragma clang diagnostic ignored "-Wmissing-prototypes"
 #pragma clang diagnostic ignored "-Wdeprecated-non-prototype"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
 #endif
 #include <stdlib.h>
 
@@ -502,4 +506,6 @@ int  _K4Search_CheckObstructionIntegrity(graphP theGraph, graphP origGraph)
 
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
diff --git a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphOuterplanarObstruction.c b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphOuterplanarObstruction.c
index 848106457..9839059b0 100644
--- a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphOuterplanarObstruction.c
+++ b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphOuterplanarObstruction.c
@@ -11,6 +11,10 @@ See the LICENSE.TXT file for licensing information.
 #pragma clang diagnostic ignored "-Wstrict-prototypes"
 #pragma clang diagnostic ignored "-Wmissing-prototypes"
 #pragma clang diagnostic ignored "-Wdeprecated-non-prototype"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
 #endif
 
 /* Imported functions */
@@ -203,4 +207,6 @@ isolatorContextP IC = &theGraph->IC;
 }
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
diff --git a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphTests.c b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphTests.c
index c9124cc03..c8fe95d3c 100644
--- a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphTests.c
+++ b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphTests.c
@@ -9,6 +9,10 @@ See the LICENSE.TXT file for licensing information.
 #pragma clang diagnostic ignored "-Wstrict-prototypes"
 #pragma clang diagnostic ignored "-Wmissing-prototypes"
 #pragma clang diagnostic ignored "-Wdeprecated-non-prototype"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
 #endif
 
 #define GRAPHTEST_C
diff --git a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphUtils.c b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphUtils.c
index 5bb906dd3..1cc18bce7 100644
--- a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphUtils.c
+++ b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/graphUtils.c
@@ -9,6 +9,11 @@ See the LICENSE.TXT file for licensing information.
 #pragma clang diagnostic ignored "-Wstrict-prototypes"
 #pragma clang diagnostic ignored "-Wmissing-prototypes"
 #pragma clang diagnostic ignored "-Wdeprecated-non-prototype"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
+#pragma GCC diagnostic ignored "-Wold-style-definition"
 #endif
 
 #include <stdlib.h>
@@ -2501,4 +2506,6 @@ int debugNOTOK()
 }
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
diff --git a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/planarity.h b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/planarity.h
index 7f2ad40a5..4b305d296 100644
--- a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/planarity.h
+++ b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/planarity.h
@@ -6,6 +6,10 @@
 #pragma clang diagnostic ignored "-Wstrict-prototypes"
 #pragma clang diagnostic ignored "-Wmissing-prototypes"
 #pragma clang diagnostic ignored "-Wdeprecated-non-prototype"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
 #endif
 /*
 Copyright (c) 1997-2020, John M. Boyer
diff --git a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/planarityRandomGraphs.c b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/planarityRandomGraphs.c
index adf2b1e6c..fb96b59a4 100644
--- a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/planarityRandomGraphs.c
+++ b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/planarityRandomGraphs.c
@@ -11,6 +11,13 @@ See the LICENSE.TXT file for licensing information.
 #pragma clang diagnostic ignored "-Wdeprecated-non-prototype"
 #pragma clang diagnostic ignored                                             \
     "-Wincompatible-pointer-types-discards-qualifiers"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
+#pragma GCC diagnostic ignored "-Wdiscarded-qualifiers"
+#pragma GCC diagnostic ignored "-Wswitch-default"
+#pragma GCC diagnostic ignored "-Wunused-result"
 #endif
 #include "planarity.h"
 
@@ -488,4 +495,6 @@ int RandomGraph(char   command,
 }
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
diff --git a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/planaritySpecificGraph.c b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/planaritySpecificGraph.c
index c6089ddeb..cc9cf2f73 100644
--- a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/planaritySpecificGraph.c
+++ b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/planaritySpecificGraph.c
@@ -10,6 +10,12 @@ See the LICENSE.TXT file for licensing information.
 #pragma clang diagnostic ignored "-Wmissing-prototypes"
 #pragma clang diagnostic ignored "-Wdeprecated-non-prototype"
 #pragma clang diagnostic ignored "-Wincompatible-pointer-types-discards-qualifiers"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
+#pragma GCC diagnostic ignored "-Wswitch-default"
+#pragma GCC diagnostic ignored "-Wdiscarded-qualifiers"
 #endif
 #include "planarity.h"
 
@@ -183,4 +189,6 @@ void WriteAlgorithmResults(graphP theGraph, int Result, char command, platform_t
 }
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
diff --git a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/planarityUtils.c b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/planarityUtils.c
index a975ee707..f8829240b 100644
--- a/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/planarityUtils.c
+++ b/extern/edge-addition-planarity-suite-Version_3.0.1.0/c/planarityUtils.c
@@ -11,6 +11,14 @@ See the LICENSE.TXT file for licensing information.
 #pragma clang diagnostic ignored "-Wdeprecated-non-prototype"
 #pragma clang diagnostic ignored                                             \
     "-Wincompatible-pointer-types-discards-qualifiers"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
+#pragma GCC diagnostic ignored "-Wold-style-definition"
+#pragma GCC diagnostic ignored "-Wdiscarded-qualifiers"
+#pragma GCC diagnostic ignored "-Wswitch-default"
+#pragma GCC diagnostic ignored "-Wunused-result"
 #endif
 #include "planarity.h"
 
@@ -414,4 +422,6 @@ char *ConstructPrimaryOutputFilename(char *infileName, char *outfileName, char c
 }
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
diff --git a/src/bliss-includes.h b/src/bliss-includes.h
index bf049d8bd..911efa9bb 100644
--- a/src/bliss-includes.h
+++ b/src/bliss-includes.h
@@ -20,6 +20,9 @@
 #if defined(__clang__)
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wformat-nonliteral"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wformat-nonliteral"
 #endif
 
 // GAP headers
@@ -37,6 +40,8 @@
 
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
 
 #endif  // DIGRAPHS_SRC_BLISS_INCLUDES_H_
diff --git a/src/digraphs.c b/src/digraphs.c
index 2f20c3999..1fb90ce39 100644
--- a/src/digraphs.c
+++ b/src/digraphs.c
@@ -1140,7 +1140,8 @@ static Obj FLOYD_WARSHALL(Obj digraph,
                           bool copy,
                           bool diameter,
                           bool shortest) {
-  Int n, i, j, k, *dist, *adj;
+  Int n, i, j, k, *dist;
+  Int* adj = NULL;
   Obj next, out, outi, val;
 
   n = DigraphNrVertices(digraph);
@@ -1204,9 +1205,7 @@ static Obj FLOYD_WARSHALL(Obj digraph,
           maximum = dist[i * n + j];
         } else if (dist[i * n + j] == -1) {
           free(dist);
-          if (copy) {
-            free(adj);
-          }
+          free(adj);
           return Fail;
         }
       }
@@ -2370,6 +2369,9 @@ static StructInitInfo module = {
 #if defined(__clang__)
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wmissing-prototypes"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wmissing-prototypes"
 #endif
 StructInitInfo* Init__Dynamic(void) {
   return &module;
@@ -2380,5 +2382,7 @@ StructInitInfo* Init__digraphs(void) {
 }
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
 #endif
diff --git a/src/gap-includes.h b/src/gap-includes.h
index b824cc497..5277bf4b8 100644
--- a/src/gap-includes.h
+++ b/src/gap-includes.h
@@ -18,6 +18,11 @@
 #if defined(__clang__)
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wdeclaration-after-statement"
+#elif defined (__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wdeclaration-after-statement"
+#pragma GCC diagnostic ignored "-Wpedantic"
+#pragma GCC diagnostic ignored "-Winline"
 #endif
 // GAP headers
 #include "compiled.h"  // for Obj, Int
diff --git a/src/planar.c b/src/planar.c
index 373ef2677..b53355416 100644
--- a/src/planar.c
+++ b/src/planar.c
@@ -27,6 +27,10 @@
 #pragma clang diagnostic ignored \
     "-Wincompatible-pointer-types-discards-qualifiers"
 #pragma clang diagnostic ignored "-Wstrict-prototypes"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-prototypes"
+#pragma GCC diagnostic ignored "-Wswitch-default"
 #endif
 #ifdef DIGRAPHS_WITH_INCLUDED_PLANARITY
 #include "c/graph.h"
@@ -41,6 +45,8 @@
 #endif
 #if defined(__clang__)
 #pragma clang diagnostic pop
+#elif defined(__GNUC__)
+#pragma GCC diagnostic pop
 #endif
 
 // Forward declaration of the main function in this file.
@@ -143,6 +149,8 @@ Obj boyers_planarity_check(Obj digraph, int flags, bool krtwsk) {
     case EMBEDFLAGS_SEARCHFORK4:
       gp_AttachK4Search(theGraph);
       break;
+    default:
+      break;
   }
   if (gp_InitGraph(theGraph, V) != OK) {
     gp_Free(&theGraph);