First-principles calculations were performed to investigate the interaction of small gas molecules, including H-2, O-2, H2O, NH3, NO, NO2, and CO, with monolayer GaSe. The energetics, charge transfer and band structures were evaluated by considering the Grimme-D2 correction. Due to the low adsorption energies and the moderate charge transfer, monolayer GaSe could be a promising candidate as a sensor for O-2 and NO2. The theoretical results for adsorption of O-2 on monolayer GaSe are consistent well with the most recent experimental observation. Diverse projected band structures of these gas molecule adsorbed systems demonstrate that there exist two kinds of charge transfer mechanisms: traditional and orbital mixing theories. Based on the proposed charge transfer mechanisms, external strain exerted different influences on the charge transfer for gas molecules adsorbed on the GaSe monolayer. The present study provides theoretical insight leading to a better understanding of the novel two-dimensional materials, such as graphene, phosphorene, and transition metal dichalcogenides. (C) 2017 Elsevier B.V. All rights reserved.