• The GDL Reference Guide. They don't give us a printed version anymore. That's OK, the PDF is better. This guide is a really bad tutorial, because it's not a tutorial. It's more like a dictionary. Once you have the basic ideas, it's very clear, and indispensable.
• The GDL Cookbook 3, by David Nicholson-Cole. PDF. Out of print, out of date, but still useful. How to do lots of things. What most of the commands do. A bunch of tips. Handy references for binary arithmetic and circle geometry. Inspirational objects by people smarter than us, such as fountains, waving flags, ferris wheels. David is the primary evangelist for GDL and Archicad culture in general, one of the pillars of the community, and also the author of...
• Object Making with Archicad: GDL for Beginners
• The Libraries / Library Parts / GDL forum of Archicad Talk. Like any question you might consider asking on ACTalk, ask me first, it's faster.
• GDL Talk on Yahoo Groups. Same advice as above, with the added caveat that issues in this forum tend to be of a higher level.
• The File -> GDL Objects -> Open Object command. GDL scripts are de facto open, and it's a good way to see applied examples. A word of advice: It's also a good way to see bad scripting, especially in the Archicad library itself. Anyway, no one can stop you from reading a script and trying to figure it out.
• Your humble narrator. On a scale of 1 to 10, I'm a seven. I don't know everything, but I know of everything. I've done tons of it, over the course of five years. I've done a couple of things first, maybe. I can't teach you all I know due to time constraints, but I can give guidance on basic to intermediate questions.
All the PDFs mentioned here are available at 3 Resources : Documentation. Remember, PDFs in your local Archicad 9 : Documentation folder will show up in the help menu within Archicad. So handy.
I strongly, strongly, strongly recommend opening all PDFs in Preview, the reader by Apple that comes with OS X, rather than Acrobat Reader by Adobe. It's faster, simpler, launches quicker, and has better searching. No comparison.