The AC does use gas. Once the engine is switched on, it draws the power from the combustion inside the engine. The alternator continuously charges the engine for the next start up. The AC draws power from the alternator, which draws power from the engine. The fuel powers the engine.
Though the amount of gasoline used due to AC usage is negligible, it can build up over time. This is especially significant for those who often travel, and in areas with extreme weather conditions. However, various fundamental factors govern the gas usage, including wind resistance, engine power, weight and aerodynamics; this boils down to the car’s age and make. Note that the newer car models feature compensatory features that ensure that the gas usage due to AC use remains negligible.