The sensing properties of GeP monolayer to small gas molecule (CO, CO2, O2, H2O, NH3, NO and NO2) have been investigated by performing first-principle calculations. Based on calculated adsorption distance, adsorption energy and charge transfer, we find that GeP monolayer is the most sensitive to NO2 and NO molecules, and all gas molecules act as acceptors to gain electrons from GeP monolayer. The adsorption energy of NO2 is smaller than that of NO molecule, which implies that NO2 is more easily desorbed from GeP. And different work function changes also indicate the possibility to identify NO2 and NO in an oxygen-free environment. Moreover, we apply uniaxial and biaxial strains to GeP monolayer and find that a small biaxial compressive strain can effectively enhance the sensitivity of work function type sensor based GeP to NO2 molecule without increasing recovery time. We also discuss the effect of external vertical electric field on the adsorption performance of NO2 molecule. The negative electric field can significantly increase the adsorption strength, charge transfer and work function change of NO2-GeP system. Our results indicate that GeP monolayer is a potential candidate for NO2 gas sensor.