Magnetic nanocrystals embedded in a semiconducting matrix are gaining increasing attention for potential applications in spintronic devices. We report about the magnetic behavior of Fe and Mn doped GaN samples, fabricated by means of metal organic vapor phase epitaxy, featuring a planar array of γ'-GaxFe4-xN nanocrystals embedded in the GaN matrix. We consider a set of three samples grown with the same nominal Fe content and different Mn concentration, including one with no Mn. In the sample with the highest Mn content, we detect Mn in the γ'-GaxFe4-xN lattice and also the presence of ε-Fe3N nanocrystals. The samples exhibit a paramagnetic signal, ascribed to the GaN matrix, and a ferromagnetic one given by the nanocrystals: the former increases on increasing the Mn co-doping whereas the latter decreases. In the three samples, magnetically relaxing nanocrystals coexist with non-relaxing ones and dipolar magnetic interactions strongly affect the magnetothermal behavior. The analysis of these complex magnetic phenomena is correlated to the structural and compositional properties of the nanocrystals and to their arrangement into the GaN matrix, opening new perspectives for tuning on demand the magnetic response of this relevant system.