Although the metal-decoration can enhance the sensing properties of two-dimensional (2D) materials, the cyclic utilization of materials is hindered by the clustering tendency of metal atoms. Furthermore, there exists a risk of explosion of combustible gases with the electrical measure. Based on first-principles calculations, we investigate the adsorption of various gas molecules (O2, NO, NO2, NH3, N2, CO, CH4 and CO2) on the 2D Cu-Si extended system (Cu2Si). The NOx molecules are chemisorbed on the Cu2Si monolayer, while other gas molecules (except CH4 and N2) are held by an interaction intermediating between the physisorbed and chemisorbed states. The strong hybridizations between N 2p and Si 3p (Cu 4p) orbitals lead to the large adsorption energies. Interestingly, the adsorption of NOx (1 µB) and CO2 (2 µB) can induce magnetic moments on the intrinsically nonmagnetic Cu2Si monolayer. The magnetic moment of NO–Cu2Si mainly arises from the molecule, while the magnetic moments for the NO2 and CO2 adsorption almost origin from the monolayer. In addition, an antiferromagnetic coupling is found in CO–Cu2Si. The changes in magnetization upon the gas adsorption may be detected sensitively and safely, suggesting the Cu2Si monolayer is potential for gas sensing.