Apparently, you invoke gcg as follows:

gcg -I<include-path> -o colormap_dialog.c colormap_dialog.gc

The README I found follows, but seems to be out of date:


GCG - docs forthcoming (hopefully)

GCG is under development, it doesn't do much of anything sensible yet. What
little it does, you can test with:

gcg -t gimpimageT.h -f gimpimage.h -p gimpimageP.h -s gimpimage.c gimpimage.def