Wurtzite GaN films are grown by low-pressure MOCVD on (0001)-plane sapphire substrates. The GaN films have a total thickness of 4 $mu$m with a surface Mg-doped p-type layer, which has a thickness of 0.5 $mu$m. 90k eV $Mn^{+}$ ions are implanted into the GaN films at room temperature with doses ranging from $1 imes10^{15}$ to $1 imes 10^{16} extrm{cm}^{-2}$. After an annealing step at $770^{circ}C$ in flowing $N_2$, the structural characteristics of the $Mn^{+}$ implanted GaN films are studied by X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS) and atomic force microscopy (AFM). The structural and morphological changes brought about by $Mn^{+}$ implantation and annealing are characterized.