To use this plugin with PhoneGap Build, you must use the github source:
<gap:plugin spec="https://github.com/mapsplugin/cordova-plugin-googlemaps" source="git">
<param name="API_KEY_FOR_ANDROID" value="XXXXXXXXXXX" />
<param name="API_KEY_FOR_IOS" value="XXXXXXXXXXXXX" />
</gap:plugin>
Reference: mapsplugin/cordova-plugin-googlemaps#1062