The single GG with which I had long-term experience...it was a single because it quickly killed the other two I got with it, when all three were around an inch in length...lived for many years with almost no aggression displayed to other species. The only exception to this was a Red Devil of similar size. I simultaneously added the Devil and the GG to a mixed community in a 360-gallon tank and the Devil was brutally attacked by the GG almost immediately; I have no doubt it would have been dead if left overnight. Other fish in the tank...Oscars, large catfish, a Jardinii arowana...were completely ignored.
My GG also lived for an extended period in a large tank which it shared with large numbers of barbs, tetras, small livebearers, etc. and it never even glanced at them; a stately giant gliding through schools of tiny bite-size fish which I believe were simply too small to bother chasing. It made a pretty picture.
GG's seem to be one of those species that display a wide range of "personalities"; largely a matter of luck as to whether you get a pussycat or a psychopath.