There are many places on the Web. You can start with the GDL Depository at the bottom of the ArchicTalk main page.
Also if you go to the ArchicadWiki there is a page with many links.
http://www.archicadwiki.com/LinkCollection
Erich
AC 19 6006 & AC 20
Mac OS 10.11.5
15" Retina MacBook Pro 2.6
27" iMac Retina 5K