We use density functional theory to evaluate the metals adsorption on the surface of Gallium Nitride (GaN) nanowire (NW) and propose a model to explore H-2 sensing enhancement mechanism of metal-functionalized GaN nanowire in theory. The adsorption energy is negative when the metal atoms move closed to the surface of GaN NWs, which reflect the feature of exothermic reaction. Moreover, the simulation indicate the metal-functionalized GaN have much stronger sensing to H-2 by forming chemisorption between H-2 molecules and metal atoms on the surface, rather than the conventional van der Waals forces between H-2 molecules and the pure GaN surface. The latter can only lead to a poor response to H-2 since the weak interactions. Comparing with Au, Ag and Cu-functionalized surface, H-2 molecules are adsorbed at N-functionalized surface, with the lowest adsorption energy, which indicate it is expect to be the most suitable for H-2 detection. It should be originated from the largest band gap change and most significantly surface charge reconstruction of Pt-functionalized surface. Our results present an enhancement mechanism of gas sensing from the reduction of the surface potential by the metals effect, which can be applied to design and advance gas-sensing materials.