Predictive calculations based on density functional theory (DFT) are used here to study the electronic and optical properties of GeSe monolayer after adsorbing gas molecules (O₂, NH₃, SO₂, H₂, CO₂, H₂S, NO₂, CH₄, H₂O, NO, CO). Our results reveal that for all the gas molecules considered, only NH₃ is adsorbed on GeSe monolayer by physisorption. Whereas SO₂ and NO₂ are chemisorbed on GeSe monolayer with strong adsorption energies. In addition, the adsorption of O₂, NO and NO₂ distinctly enhances the optical absorbance and broaden the absorbance range of GeSe monolayer in visible light region. Also, it is found that the adsorption of H₂S, NO and NH₃ can reduce the work function of the GeSe monolayer. The results indicate that GeSe monolayer is not only a promising candidate for the sensing, capture, and storage of NH₃, but also an anticipated disposable gas sensor or metal-free catalyst for detecting and catalyzing SO₂ and NO₂. Furthermore, it has excellent potential to be applied to optical sensors, solar cells, nanoelectronics or optoelectronics devices.