As Barry said, all icons are embedded in the program itself. Extracting them is a question for the Developer forum, vs this Library forum. The actual resources are there, but accessible only by those who know something about the developer tools. The resolutions usable for a GDL object's UI are quite different from the scaleable icons for an app.
One of the forum moderators • AC 25 USA and earlier • MacOS 11.6.2, iMac Pro
I´m aware that programming those icons in ArchiCAD is something totally different (on the programming level) from those used by GDL, but graphical designers usually use Vector Graphics Design tools (for example: Adobe Illustrators & similar) to provide the graphic for the program.
I will ask the developer forum for those files
A good friend of mine have once told me that I´m so brute that I´m capable of creating a GDL script capable of creating GDLs.