You should be able to drag and drop 2d elements into the 2d script field pretty reliably. Make sure you have the ArchiCAD 2d window floating next to the Library Object editor with little or no space in between. You need to fling the 2d object into the 2d script, ie drag it fast: a slow drag will get stopped at the border.
Dragging 3d elements into the 3d script is another can of worms. This has always resulted in pretty bewildering GDL and I think in recent versions this has gotten a little screwier.
I've mostly dragged simple 2d fills or polylines into the 2d script window and then adapted that script to make simple 3d objects like PRISM_, CPRISM_, TUBE, REVOLVE etc.
If you really need to see the 3d script of something in ArchiCAD you can always File/Library and Objects/ Save Selection As/ Object. Then take a look in the GDL object editor. This is sometimes useful and always educational.
David Collins
Win10 64bit Intel i7 6700 3.40 Ghz, 32 Gb RAM, GeForce RTX 3070
AC 27.0 (4001 INT FULL)