The calculations of two - dimensional material structure of germanene and the adsorption of gas molecules (such as CO, CO2, N2) on germanene are performed by using density functional theory. The results show that germanene has zero-band gap energy and non-spin polarized characteristics. The adsorption of gas molecules on the surface of germanene plays a role in the tunable band gap. The adsorption energy of CO is lower than that of CO2 and N2, indicating that CO is more easily to be absorbed by the other two gases. This result makes germanene a potential candidate for CO adsorption devices, contributing to the reduction of the amount of CO in the air to create a clean beautiful environment.