Group call forwarding is not equal extension forwarding in neither PBXs. The individual extension is not due for forwarding group calls. That's true the 3cx doesn't have GCFW function but it is not a serious imperfection.
For me it would be nice if I could have my mobile be a part of the ring group since I am regularly in and out of office, and it is a pain to keep making changes to direct calls. However I would not criticize 3CX for this inability.