The bulk damage of p-type silicon sensors caused by gamma irradiation with high total ionizing doses has been investigated. The study was carried out on different types of n+-in-p silicon diodes with different oxygen concentrations and silicon bulk resistivities. The Secondary-Ion Mass Spectrometry technique was used to determine the relative concentration of oxygen in the individual samples. The measured diodes were irradiated by a 60Co gamma source to total ionizing doses ranging from 0.50 up to 8.28 MGy, and annealed for 80 min at 60 °C. The main goal of the study was to characterize the gamma-radiation induced displacement damage by measuring I–V and C–V characteristics, and the evolution of the full depletion voltage with the total ionizing dose. The Transient Current Technique was used to verify the full depletion voltage and to extract the electric field distribution and the sign of the space charge in the silicon diodes irradiated to the lowest and the highest delivered total ionizing doses. The results show a linear increase of the bulk leakage current with the total ionizing dose, with the damage coefficient being dependent on initial resistivity and oxygen concentration of the silicon diode. The effective doping concentration and full depletion voltage decrease significantly with an increasing total ionizing dose, before starting to increase again at a specific dose. We assume that the initial decrease in the effective doping concentration is caused by the effect of acceptor removal. An additional notable finding of this study is that the bulk leakage current and C–V characteristics of the gamma-irradiated diodes do not show any evidence of an annealing effect.