Uhh dude, we're not talking about a Red Devil or an Oscar here, GT's dont get THAT big. You can put several other fish in a 50g with a GT. I Have a GT, and Ill list what I have in there with it, granted I have a 125G tank so you wont be able to stock as many fish as Ive got but Im posting my list just so you can see what is compatable with a GT.
Red Devil
Green Terror
2 Jack Dempseys
3 Convicts
14 African cichlids/Mbuna's
Black Shark
Red Tail Black shark
Leporinus
The Devil and Black shark would be too big for a 50G, however, Dempseys do really well with GT's as do Convicts as long as you go with only males so they dont start pairing off. African cichlids work well, and unlike CA/SA cichlids, its ok to overstock africans as it spreads out agression. If you are looking for non cichlid tankmates, those can be tough to find as cichlids will kill most "other" types of fish, that said, Red Tail Sharks and Leporinus are among the few "other" types of fish that mix REALLY well with cichlids. So it just depends on what you want to do. I think you could Get a GT, a Dempsey, a leporinus or a Red Tail shark and several Africans and I think it would look great and work out well for you! Best of luck! I definantly agree that a cannister filter is a MUST!!! And Sand looks WAY better than gravel! Just be sure that if you go with sand, make sure your water intake tube from the cannister filter does not sit very deep in the water, you want to keep it kinda high up, even if you have to rig it so it sits right because with sand, if it is to close to the sand, the filter will suck it up and it will destroy the impellor in the cannister and that would be bad!