Pristine two-dimensional (2D) GaN is non-magnetic, which limits its direct use in semiconductor spintronics. Here, first-principles calculations were used to screen 3d transition-metal substitution (TM = Sc-Zn) at 6.25 at.% and 9.375 at.% and to evaluate biaxial strain from \(-3%\) to \(3%\). Co substitution produced the most favorable magnetic semiconductor response among the screened dopants. The two concentrations yielded magnetic moments of \(8.00\) and \(12.00\) \(\mu_B\) per supercell and band gaps of \(0.80\) and \(0.55\) eV, respectively. Biaxial strain preserved the magnetic order while tuning the band gaps. In the \(6.25\) at.% Co-doped system, the gap reached \(0.89\) eV at \(2%\)... More >