High-quality transmutation doping of GaN was demonstrated utilizing thermal neutrons to convert the Ga into Ge. Doping concentrations of 1016–1018 Ge atoms/cm3 were achieved in bulk GaN wafers. Gamma-ray spectroscopy was used to measure the doping concentrations, and SIMS analysis was used to verify the superior spatial doping uniformity in the crystal. A nitrogen-rich thermal annealing process was used to mitigate radiation-induced damage. Carbon-14 was also generated by the transmutation of the N. The 14C production rate changed drastically by adjusting the neutron energy level. Photoluminescence (PL) spectrum data indicates 14C may exhibit p-type dopant behavior due to the higher peak of the redshifted yellow luminescence (YL) band. Selective area doping was achieved using gadolinium shadow masks with directed neutrons. The defect-curing effects of fast neutrons were investigated using single-crystalline GaN samples exposed to four distinct neutron fluences. At specific irradiation conditions, the number of cured defects was more than the number of freshly produced defects, and some defects that were generated during the growth of GaN crystals were cured at a low neutron fluence. Other radiation sources, including proton and gamma beams, were also investigated.