In the context of the pursuit of a dilute magnetic semiconductor for spintronic applications, a set of GaMnN samples with varying Mn concentration and Si or Mg co-doping was investigated by optical and electron spin resonance spectroscopy. The results clearly demonstrate how the charge state of Mn is changed between 2+, 3+ and 4+ by Mg and Si co-doping. For p-type GaMnN we show that the introduction of the Mn3+/4+ donor can be compensated by Mg co-doping lowering the Fermi energy below the Mn3+/4+ level. While our results are in agreement with the hypothesis that the infrared photoluminescence appearing in GaMnN upon Mg doping originates from Mn4+, an unambiguous proof is still to be presented. Under this assumption, our measurements show that the Mn4+ center must be excited via an extra-center process at 2.54 eV.