Strongly seconded on all points. I would add:
GDL does not even support everything that is possible in Archicad input. For example, no three point arc. I never dreamed my job would involve so much circle geometry.
The environment does not even allow you to see large parts of GDL functionality: Cutting, group operations, graphical editing, 3D hotspots, and I’m sure there are others.
My hope is that this forum is the beginning of a serious recognition of GDL’s importance. The next step is to make the tools and environment less miserable.