A gas sensor comprises a basic part and a sensing layer deposited on the basic part. The basic part includes a circuit board and at least one surface acoustic wave element disposed on the circuit board. The sensing layer is a nanocomposite film of reduced graphene oxide/tungsten oxide/polypyrrole deposited on the surface acoustic wave element. The sensing layer combines reduced graphene oxide, metal oxide, and conductive polymer, so that the sensing layer is able to perform sensing at room temperature, and can be more sensitive. The present invention provides a method for manufacturing a gas sensor, and a gas sensing system including the gas sensor.