I would start with the GDL Cookbook available here
www.nottingham.ac.uk/~lazwww/cookbook/ There is also a new version around but this one works just fine and I think is a bit more complete.
Of course there is the GDL reference manual that ships with ArchiCAD (it is a pdf file found under the help menu). Eventually you will find this quite useful but at the start it will likely only confuse you. The Cookbook is a better start IMO.
Andrew Watkins excellant GDL Handbook, available from the Cadimage website -
www.cadimageworld.com/products/gdlhandbook - it a top notch reference. He explains many things not found elsewhere. If you really want to learn GDL I would highly recommend it. However, I would not start with it at the very first. Get your feet wet with the free options first then really dive in with the Handbook.
There are also various tutorial around on the web but the above resources will certainly get you going.
Oh, one last thing. Open parts all the time to see how they work. The standard ArchiCAD parts may not be the best code to look at to learn as it can get fairly convoluted. But some of the parts in the Object Depository, or from such places as Oliver Detan's web site, found here
archilib.od.pagesperso-orange.fr/, or James Murray's site, here
www.onland.info/, offer free parts that can be excellent learning tools. And of course if in doubt just search the forum here. Your question has probably already been asked and answered, and if not, you can always ask.
Erich
AC 19 6006 & AC 20
Mac OS 10.11.5
15" Retina MacBook Pro 2.6
27" iMac Retina 5K
