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 a male GT with the following:
clown loaches
bristlenose plecos
sailfin plecos
parachromis loisellei only one aggressive fish per tank unless a very massive community with several dithers
gold severum
oscar oscar will get ass whooped when the GT gets 6"
firemouth
convicts
banded rainbow fish
uaru A.
midas Far too much aggression
senegal bichirsMy GT harasses my delhezi a lot
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