Novel flexible NO2 gas sensors were fabricated by covalently bonding graphene oxide (GO) to a gold electrode on a plastic substrate using a peptide chemical protocol and then reducing in situ GO film to a reduced GO (RGO) film. A pair of comb-like Au electrodes on a polyethylene terephthalate (PET) substrate were pretreated with cysteamine hydrochloride (CH) and then reacted with GO using N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) as the peptide coupling reagent, before undergoing a final reduction by sodium borohydride (NaBH4). The anchored RGO film was characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS) and Fourier transform infrared spectroscopy (FTIR). The gas sensing properties, including sensitivity, sensing linearity, reproducibility, response time, recovery time, cross-sensitivity effects and long-term stability, were investigated. Interfering gas NH3 affected the limit of detection (LOD) of a target NO2 gas in a real-world binary gas mixture. The flexible NO2 gas sensor exhibited a strong response and good flexibility that exceeded that of sensors that were made from graphene film grown by chemical vapor deposition method (CVD-graphene) at room temperature. Its use is practical because it is so easy to fabricate.