I'd probably start with something a little simpler if you have not done GDL scripting before.
I still think the best source for beginners is the GDL Cookbook by David Nicholson-Cole.
Here is a link to an online version of Cookbook 3.
http://issuu.com/dnicholsoncole/docs/gdlcookbook3_01/305
It is old (written for version 7) but it is still very relevant to GDL today.
If you Google a bit you may find a version in PDF to download.
You will also find a Cookbook 4 which explains newer features but I don't think this has ever been completed.
There are other sources.
The GDL Reference manual in the HELP menu of Archicad (free).
And CADimage have a GDL Handbook (or they used to) but that will cost you a bit.
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11