An optical gas sensor was prepared by depositing graphene oxide flakes over a monolayer of gold nanoparticles, chemically attached to a functionalized fused silica substrate. The coupling between flakes and nanoparticles lead to optical changes upon exposure to different gases: in particular, we observed a shift of the surface plasmon resonance band in presence of both reducing and oxidizing gases. This effect can be explained in terms of a strong gold–graphene interaction and specifically of electron transfer between the gold nanoparticles and the two-dimensional sheet of the sp2-hybridized carbons of graphene oxide.