Exotic properties are predicted to be functionalized by the adsorption of non-magnetic Ge atom on the two-dimensional GaN (2D–GaN) monolayer. They include the existence of spin filtering, high magnetism and continuous emission of electromagnetic radiation. A comprehensive ab-initio study of structural, electronic, magnetic and optical properties of 2D–GaN monolayer possessing several Ge atoms as adsorbents has been performed. We employ the norm-conserving pseudopotentials for atoms and plane waves in a generalised gradient approximation within density functional theory (DFT). The semiconducting pure GaN monolayer is converted into a metallic one in far 2-Ge atoms adsorbed above Ga in 2D-GaN monolayer systems. The behaviour of magnetism is found to be site-independent. The binding energies per adatom of the Ge-atoms adsorbed above N are larger than those of the Ge-atoms adsorbed above Ga atoms. Along with the main strong absorption lying in the ultraviolet region in all the Ge-adsorbed GaN monolayers, strong continuous absorption from the far infrared region to the visible region has been seen in some systems. These systems may be useful for development of light-emitting devices that will emit in various regions of the energy spectrum including visible region. The emission in the deep ultraviolet region may also be used for sterilization, water purification, etc.