<p>We have employed first-principles DFT calculations within the GGA + <i>U</i> framework to investigate GaN containing impurity Mn inclusion co-existing with monovacancies of Gallium (Ga), Nitrogen (N), and Ga-N divacancies, i.e., V_Ga, V_N and V_Ga-_N respectively. Ga and Ga-N vacancies, i.e., V_Ga and V_Ga-_N introduces half-metallicity while V_N is metallic. Materials exhibiting half-metallic properties are suitable for spin-filtering application. Agglomerated Mn atoms prefer the ferromagnetic (FM) configuration. Also, N vacancy enhances the formation of Mn in GaN and there is greater possibility of Mn inclusion coexisting with the N vacancies rather than Ga vacancies. Ga-N divacancy (V_Ga-_N) reduces the magnetic moment of a single substitutional Mn (Mn_Ga) while having no significant effect on a small Mn cluster of 2Mn_Ga. Regarding Mn_Ga-vacancy complexes, V_N and V_Ga-N may change the sign of exchange energy (<i>J</i>) in a small 2Mn_Ga cluster such that for 2Mn_Ga-V_N and 2Mn_Ga-V_Ga-_N defect complexes, the magnetic configuration changes from FM to antiferromagnetic (AFM) as evidenced by the change in the sign of<i> J</i> in the respective defect complexes. Density-of-states analysis of 2Mn_Ga-V_N and 2Mn_Ga-V_Ga-_N defect complexes shows the shifting of major energy peaks away from Fermi energy level. This indicates less bonding and thus less stable magnetic configuration which thus favors their respective AFM configurations. Our study shows that introducing intrinsic vacancy defects and metal ions-vacancy complexes in GaN can be used to tune its electronic and magnetic properties and should stimulate experimental and theoretical broad studies.</p>