There is more than one of these giant gobies sold as aquarium fish. These species are here in
the Brackish FAQ as well as some information on their maintenance requirements.
Even the smallest species, Gobioides peruanus, gets to around 30 to 40 cm long in captivity. Other species are appreciably bigger. While you might be able to keep a single specimen in an over-filtered 40 gallon tank, realistically you want something quite a bit bigger.
As others have said, they are peaceful. They may eat small fish (like guppies) or shrimps
if at the absolute point of starvation. But they prefer small invertebrates (such as brine shrimps), bloodworms, algae, and various dried foods like catfish pellets. Algae is important for good health. They have tiny teeth used to rasp algae from stones and mud, so provide them with a substitute, such as pellets or wafers produced for plecs.
All the Gobioides spp. gobies and their relatives need a sandy aquarium. Silver sand or coral sand are ideal. Do not keep them in a tank with gravel if you want the fish to be happy and long-lived. You will also need tubes of some kind as burrow substitutes (these fish normally live in mud burrows). PVC tubes are ideal. They are territorial, but if not overcrowded, they do not harm one another.
While these are actually astonishingly hardy fish, they have been sadly abused over the years, and most specimens appear to die a premature death. Kept well, they are real characters, ugly, but lovely nonetheless.
Cheers,
Neale