A while back, I created a three-part video tutorial series on Creating and Customizing Library Parts showing how to do some light GDL editing to create custom library parts:
Gary Lawes is currently teaching an online GDL course as part of the Masters of ArchiCAD Training Series:
Practical, Parametric Object Making with ArchiCAD GDL
For more information about this 4 lesson course (which is being presented live via GoToWebinar, with recordings made available for viewing on demand), please visit: http://www.mastersofarchicad.com/gdl
I hope these educational resources may be added to this GDL site in an appropriate location.
We can put your site to the GDL sites section. Please send a mail about your site to firstname.lastname@example.org, give a short description about your site, and send us a logo. As we receive this request, we will handle it soon.
Team Leader, Library Team GRAPHISOFT SE