The adsorption behaviors of gas molecules CH4 and CO on polar (0001), (000–1) facets and nonpolar (10-10), (11–20) facets of wurtzite GaN were explored in details by first-principles calculations. Among these important low-index facets, CH4 on the (0001) facet exhibits the lowest adsorption energy of −1.57 eV, signifying the highest density of adsorbed CH4 molecules under the same methane concentration in air. Meanwhile, the C–H bond is weakened revealed by the bond length elongation from 1.0956 Å to 1.1020 Å during adsorption, consistent with charge redistribution results. The adsorption energies for CO on the (0001) and (000–1) facets are calculated to be −2.17 eV and −3.89 eV, much lower than those on nonpolar facets. Obviously, the corresponding C–O bond is activated during adsorption, proved by the bond length elongated from 1.1531 Å to 1.2003 Å and 1.1715 Å, respectively, in line with charge redistribution results. Particularly, the co-adsorption simulation verifies the surface redox reaction between CO and O2 on + c-plane GaN. Therefore, we predict superior performance of (0001) GaN in detection for high concentration CH4 and (000–1) GaN in selective detection for low concentration CO.