Density functional theory calculations have been carried out to study the adsorption of varous gas molecules (H2O, NH3, CO, NO2 and NO) on pristine graphene and Ga-doped graphene in order to explore the feasibility of Ga-doped graphene based gas sensor. For each gas molecule, various adsorption positions and orientations were considered. The most stable configuration was determined and the adsorption energies with van der Waals interactions were calculated. Further, electronic properties such as electron density, density of states, charge transfer and band structure were investigated to understand the mechanism of adsorption. The results showed that the gas molecules studied were only weakly adsorbed on pristine graphene with small adsorption energies. On the other hand, the adsorption energies of all gas molecules on Ga-doped graphene increased by various amounts. Adsorption of gas molecules on Gadoped graphene can open a relatively large band gap ranging from 0.267 to 0.397 eV. NO2 was found to be very sensitive to Ga-doped graphene with adsorption energy of -1.928 eV due to strong orbital hybridization and large charge transfer. Furthermore, our study suggests that the affinity and electronic properties of NO2 on Ga-doped graphene can be dramatically changed by an external electric field. A negative electric field enhances the adsorption of NO2 on Ga-doped graphene as reflected in the increase in adsorption energy. In contrast, the interaction will be weakened under a positive electric field. The results of the DFT calculation indicates the potential application of Ga-doped graphene in gas sensing for NO2 detection, and the advantage to use external electric field to tune the sensitivity for NO2. (C) 2017 Elsevier B.V. All rights reserved.