I have an 18 inch GG in with Centrals and she is fine, there are a few squabbles (which she always wins) but no major aggression. The tank is 9ft x 30in x 24in but i would be less sure of it working in a smaller tank.
Good luck
They're bloody tough fish, and they seem quite docile and slow once they get large. I've seen them kept with Peaceful SA's and aggressive CA/SA's like Red Terrors, Devils, Texas etc. without any problems.