Epitaxial GaN layers with a-, c- and m-plane surface orientations were implanted with 300 keV Ar-ions at 15 K with fluences ranging from 2 × 1012 to 4 × 1016 at/cm2. Damage build-up proceeds in three steps separated by wide fluence regions where the maximum damage level, measured by in situ Rutherford Backscattering Spectrometry/Channelling, saturates. The three steps occur at similar fluences for the three crystal orientations and similar defect formation rates for the lowest fluences are observed. Surprisingly, the second saturation regime reveals a significantly lower damage level in a-plane layers while m- and c-plane samples suffer more than 4 times higher damage levels. The strong radiation resistance of a-plane GaN was attributed to very efficient dynamic annealing of point defects during the implantation even at 15 K. The migration and aggregation of point defects also lead to distinct defect microstructures as evidenced by transmission electron microscopy. Besides point defects and their clusters the dominant extended defects caused by implantation in c-plane GaN are basal stacking faults while dislocation loops are formed in a-plane material. m-plane GaN presents a mixture of planar defects and dislocation loops after implantation.