The answer is very easy
Imagine simple GDL object - box AxBxZZYZX
Note You should enter new parameter (in addition to basic A & B) -ZZYZX (exactly the name zzyzx !!!)
The 3D script will look like this
BLOCK a,b,zzyzx
HOTSPOT 0,0,0
HOTSPOT a,0,0
HOTSPOT a,b,0
HOTSPOT 0,b,0
HOTSPOT 0,0,zzyzx
HOTSPOT a,0,zzyzx
HOTSPOT a,b,zzyzx
HOTSPOT 0,b,zzyzx
Hotspots should be placed exactly in object corner positions (a,b,xxyzx)
All dimensions will be editable from 3D view
Note that even on 2D view hotspots shoud be placed into these points - otherwise object willn't be ediotable by drag&drop
Why 'non'-graphisoft developers doesn't sometimes follow this rule ? - perhaps they simply doný know it or doesn't care about 3D window editing.
For example I done more then 300 different objects - from simple ones t overy-very complicated ones (150-200 kB program text) - but I also offen don't put featured 3D hotspots
Hope this answer will find You well