Using first-principles calculation, we have studied the adsorption effect of small gas molecules (H2O, CO2, CH4, SO2, H2S, and NH3) on GeP3 monolayer. To determine the most stable adsorption site, five adsorption sites (center, Ge, P, bridge GeP, and bridge PP) were considered in the paper. Through calculations of adsorption energy, adsorption distance, and charge transfer, we preliminarily determined that H2O, CO2, and CH4 were physically adsorbed on GeP3 via weak van der Waals force. However, SO2, H2S, and NH3 were chemically adsorbed on GeP3 with new covalent bonds formed, as concluded by calculations of electron localization function and charge density difference. Gas molecule adsorption can cause significant changes in the band gap of single-layer GeP3, indicating that pristine GeP3 monolayer is sensitive to these gases. In addition, the adsorption energy of the H2O, CO2, and CH4 adsorbed on GeP3 can be tuned effectively by employing an external electric field. Our theoretical studies reveal that GeP3 monolayer is a promising gas-sensitive material used in nanometer devices.                                                                                                     @en