I personally like the firemouth idea even though I have tiger barbs and filament barbs as dithers with my GT at the moment. The dithers are nice to add some movement though.
It needs friends, or you need to add more fish so you don't feel sorry for your poor lonely fish?
I doubt if it needs friends, I'd guessing a male GT would happily spend its life alone in a 55g. Aside from Japes suggestions, perhaps you could try a female GT? I think the tank has too small a footprint for any other cichlids to work long-term.
Uaru A. if you have the room and your GT isnt very aggressive. the GT wont need friends, its all about the keeper's preference unless you have shoaling fish.