Graphene, as a two-dimensional (2D) carbon material, has been a research focus in the field of sensors since its discovery owing to its physical and chemical properties. However, the zero-band-gap characteristic of graphene places many restrictions on its application in sensors. To expand the application potential of graphene materials in the field of detection, various surface functionalization methods have been developed. The most common and useful methods of functionalization are doping and the introduction of defects. This paper mainly reviews the state-of-the-art work on gas sensing applying defective and doped graphene. The effects of defect and heteroatom codoping on the gas-sensing properties of graphene are also discussed, and the key research directions of functionalized graphene chemical sensors in the future are proposed.