I've never understood why GPD made start, select and menu useless when not in gamepad mode.
I figured it was probably a lost cause.
But today I suddenly realised that there were three conspicuously empty slots in the configuration structure. And lo! That's what they are for!
Now gpdconfig lets you set the behaviour of start, select and menu when in mouse mode.
There's probably no more hidden functionality in there.
I think.