At low fluence, 300 keV Eu implantation in GaN leads to a strain increase followed by a saturation as observed by X-ray diffraction, while Rutherford backscattering/channeling remains insensitive to the radiation damage. Based on transmission electron microscopy, this saturation regime is attributed to a damaged region in the crystal bulk in which interaction between point defects and stacking faults (SFs) occurs, leading to the densification of the network of planar defects by the trapping of point defects. At higher fluences, above 2×10¹⁴ Eu/cm², the evolutions of strain state in another region and of the microstructure as observed by TEM indicate a modification of the degradation mechanisms which now involve a migration of point defects out of the region of SFs. This results in the formation of a highly strained area below the region of SFs made up of large point defect clusters, and in the extension of the SFs network towards the surface that eventually leads to its nanocrystallization.