At that size, these fish are difficult to identify. A lot depends on where you found it and whether it was in fresh or brackish water. Outside of the subtropics, the commonest species are Lepisosteus osseus, Lepisosteus oculatus and to a lesser degree Lepisosteus platostomus. Atractosteus spatula is really only found in the subtropics. All may be found in brackish water periodically, but Atractosteus spatula is the one that seems to favour such habitats the most, and may even be found in fully marine conditions.
For what it's worth, I suspect that this is Lepisosteus osseus. This species does sometimes have a horizontal band along its flanks. But it does have distinctly spindly jaws when compared to other gar, something that isn't entirely obvious from your photo. They're quite easy to keep as aquarium fish, though jumpers and prone to damaging their jaws if the tank is too small.