Two-dimensional (2D) GeS monolayer having a similar structure to phosphorene draws research attention due to its novel and interesting properties which may be used as energy harvesting applications. The structural defects are an efficient way to modulate the properties of the materials. In the present work, the structural, electronic and optical properties of pristine GeS, defected system with sulfur vacancy as well as oxygen substitution on the same place are studied. The S-vacancy in GeS monolayer significantly reduces the electronic bandgap and also displayed the remarkable changes in optical properties. Again, the replacement of S atom to O atom on GeS monolayer effectively maintains the electronic properties as well as optical properties which shows very close to pristine system. From these investigations, we conclude that the similar group of atoms not effectively changes the properties of layered monolayer materials and vacancy in GeS monolayer significantly changes the properties of materials which can be used as high performance nanoelectronics and optoelectronics applications.