An empty gas can only be filled with gasoline and therefore cannot be filled with water. It is possible to refill a gas can by either siphoning gas from a vehicle, or at a gas pump. It should be noted that without a source of power, a gas pump will not function.
A gas can contains gasoline which can be poured into vehicles, filling their gas tank. Along with vehicles, it is also the primary source of fuel for generators. One full gas can will refill 80% of a generator's fuel tank.
A gas can can be used in place of kindling when starting a fire, along with being added as a source of fuel to a fire. Having a gas can in the secondary slot, with a lighter in the primary, it is possible to burn corpses, removing any negative moodles that are caused by their presence.