Gas is, as implied, a gasoline station with branches located throughout San Andreas.
It is presented as a very small gas station with a Fire Extinguisher next to it. The gas station is located in Valle Ocultado, Tierra Robada next to the Tee Pee Motel. The gas station, of course, is not interactive and despite being a gas station, it doesn't have fuel lines, instead it sells gas inside. Most of the time a beater Glendale will have spawned at the back of the block.
The gas station in Angel Pine appears to be slightly larger, with gasoline pumps under a large roof. A Xoomer petroleum trailer can be found in the parking lot.