The effect of isochronal thermal annealing on Ni/n-GaN Schottky barrier diodes exposed to a total accumulated gamma-ray dose of 21 Mrad(Si) has been investigated using capacitance-voltage (C-V) and current-voltage (I-V) measurements, while capacitance deep-level transient spectroscopy (DLTS) has been employed to monitor the evolution and annihilation of radiation-induced defects during thermal annealing. Annealing temperatures up 160 °C were found to improve device I-V characteristics; however, thermal annealing above 250 °C resulted in: (a) Degradation of both forward and reverse I-V characteristics, (b) reduction in free carrier concentration, and (c) a decrease in the concentration of radiation-induced defects, as evidenced by DLTS measurements. Following annealing above 350 °C, the radiation-induced defects were no longer detectable using DLTS. Analysis of the thermally induced reduction in radiation-induced defect concentration indicated that the dominant defect-annihilation process has a mean activation energy of 1.8 eV. The physical origin of radiation-induced defects, and of defects involved in their annihilation process, is discussed in the perspective of published theoretical calculations of native defect diffusion mechanisms in GaN.