GDL Centre (the new interface is much easier to navigate) coupled with this site has taught me quite a bit on object creation.
Beware the rabbit hole is quite deep though.
If you do get into it I recommend getting an external editor that highlights syntax, I use sublime.
With regards to your manipulation of complex profiles question, I haven't come across a way to reference them in GDL (outside of the name data using a label) but there is limited functionality to stretch widths and heights within archicad.
In the complex Profile Palette there are the horizontal & vertical stretch checkboxes, they add lines to your profile that dictate the portion of it that will stretch. It's somewhat limited but could be a quick solution for you.
Also I think from memory they can only expand, so you have to draw your profile at the minimum size.
Good luck!
AC 24 5004 AUS
iMac OSX (10.13.6) 4.2ghz i7
8gb ram/8gb vram