Most people seem happy to start with the GDL Cookbook 3 by David Nicholson-Cole.
It is a bit old now but still relevant.
There is also “Introduction to Object Making with ArchiCAD: GDL for Beginners” by the same author (again a little old but still good).
Both of these are free resources.
There is also a GDL Handbook from CADimage by Andrew Watson if that is still available – but that will cost you if it is.
There is of course also the GDL Centre where you are now for this forum – follow the links at the top of this page.
Versions 6.5 to 22
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision M6800 - i7 4700MQ @ 2.40GHz, 16GB RAM, AMD FirePro M6100 (2GB), Windows 7 64bit