if the GT is male then anything smaller than a 55g is cruel. you could probably get away with a 40 long if it was a female.
although imo these are the absolute minimums, they are far from ideal. a 4ft tank begins to look tight once a fish reaches 8" and male GT's have the potential for 12"+.
i have personally kept male GT's with the following:
clown loaches
bristlenose plecos
sailfin plecos
parachromis loisellei
gold severum
oscar
firemouth
convicts
banded rainbow fish
uaru A.
midas
senegal bichirs
i have noted from various posts here and around other forums that they do particularly well with:
silver dollars
pim. pictus
as a bonded pair