If you are traveling by car you can use whatever gas is left in the tanks so long as you figure out how to pump it in.
The gas can be used as one big bomb if surrounded by zombies so long as you have some sort of explosive and can throw it. (Depending on how far the gas tanks are.) can explode and wipe out a number of the attacking zed if they are still passing the pumps. But it is not recommended without proper shielding and cover from the explosion.
Some Gas stations are double jointed to include restaurants. (Such as Jack in the Box, Subway, etc.) So there will be food inside even if it does not include a restaurant, so long as nobody has picked it clean of all food.
They are almost always in populated areas so try to avoid if possible.
Gas will be one of the first things people panic-buy in a disaster situation, so there may well be no supplies to be found.
The gas can be used as an explosive however they tend to engulf the station and set it ablaze and shatter the windows. And you can get seriously burned and rendered immobile or killed by the fire if you don't have some sort of metal to shield yourself from the fire or at least some cover, and a lot of water to clear a path.
Most tend to have big windows so it can be costly to blockade these giant windows, and bandits can easily penetrate said windows.