Chemical functionalities of graphene oxide play an important part in anchoring gas molecules and perhaps selective mechanism in gas sensor. The adsorption behavior of methane (CH4) and ammonia (NH3) gas molecules on graphene oxide epoxy (GO-epoxy) and graphene oxide hydroxyl (GO-hydroxyl) were investigated using first-principles calculations within density functional theory (DFT) method. The structural properties (bond length and adsorption distance), electronic properties (band structures and density of states), adsorption energy (Eads) and Mulliken charge transfer were calculated using generalized gradient approximation of Perdew-Burke-Ernzerhof (GGA-PBE) as implemented in DMol3 and Cambridge Serial Total Energy Package (CASTEP) computer codes. After geometrical optimization, there are slight differences in structural properties and electronic properties before and after adsorption of CH4 and NH3 on GO-epoxy and GOhydroxyl. In comparison between CH4 and NH3 on GO-epoxy and GO-hydroxyl, highest adsorption energy was calculated for NH3 adsorbed on GO-hydroxyl (Eads = -0.775 eV) indicated higher chance of charge transfer to occur compared to others structures. The overall result shows that physisorption behavior is the main interaction between CH3 and NH4 on GO-epoxy and GO-hydroxyl for gas sensors based on GO.