Based on the first-principles of density-functional theory (DFT), the effects of gas adsorption on the change in geometric stability, electronic structure and magnetic properties of graphene with anchored Co (Co-graphene) systems were investigated. A single Co adatom interacts much weaker with pristine graphene (Co/pri-graphene) than with the graphene containing a single vacancy (Co/SV-graphene). The Co dopant provides more electrons to the dangling bonds of carbon atom at defective site and exhibits more positive charges, which makes Co/SV-graphene less prone to be adsorbed by gas molecules in comparison to Co/pri-graphene. It is found that the electronic structure and magnetic properties of Co-graphene systems can be modulated by adsorbing gas molecules. Except the NH3 molecule, the adsorbed NO, SO2, CO or HCN as electron acceptors on the Co/pri-graphene can exhibit semiconducting properties. Among the gas molecules, the strong adsorption of NO molecule can effectively regulate the magnetic properties of Co-graphene systems. Moreover, the stable configuration of Co/SV-graphene is more likely to be the gas sensor for detecting NO and SO2. The results validate that the reactivity of atomic-scale catalyst is supported on graphene sheets, which is expected to be potentially efficient in the gas sensors and electronic device. (C) 2015 Elsevier B.V. All rights reserved.