Tbh on the point about creativity in a Gmod vs GTA sense any community with 700+ people in an IRL setting is going to devolve into some people taking it as Second Life RP. You can literally make yourself as a character and you'll probably blend in just as well as the white guys LARPing as hardcore gang members do on the forums.
Meanwhile you plop a self-insert into your average Gmod server and it sticks out like a sore thumb in no time. I've seen more unique and interesting characters on Gmod just because they're not constrained to the IRL setting as a result.
But the general RP quality on GTA is higher (most of the time) which is to be expected given almost any PC can run Gmod and its dirt cheap and popular with teens.