A detailed investigation of the crystallographic damage has been carried out in GaN following 300 keV rare earth ion implantation at room temperature by varying the fluence from 7×1013 to 5×1016 at/cm2. It is shown that above a threshold fluence around 2×1015 at/cm2, nanocrystallization takes place from the surface, subsequent to the formation of a planar defects network consisting of basal and prismatic stacking faults. This network starts to form at the lowest analyzed fluence mostly around the mean projected range. When the fluence increases, it propagates toward the surface, reaching it just before the on-set of the nanocrystallization. A model based on the mechanical breakdown of the GaN wurtzite structure mediated by prismatic stacking faults is proposed.