Belone species are a group of saltwater fish often called gar; in fact the name was originally applied to them and not to the freshwater (lepisosteid) gar. They are not related. Some lepisosteid gar wander into saltwater from time to time, especially Atractosteus species, and I have heard of keepers maintaining alligator gar in full marine conditions. But most lepisosteid gar are strictly freshwater fish.
Danny, as others have said spotted gar are freshwater fish and typically get to about 24" long, though wild fish sometimes get bigger than that. They are very rare in the pet trade, but the extremely similar Florida gar is quite common in the pet trade. Most Florida gar in the pet trade are captive bred in Asia. They can be kept with other large, peaceful fish.