GaN-based metal–oxide–semiconductor (MOS) devices, such as n- and p-type capacitors and inversion- and accumulation-type p-channel field effect transistors (MOSFETs), were fabricated by Mg-ion implantation and ultra-high-pressure annealing (UHPA) under 1-GPa nitrogen pressure. Even though UHPA was conducted at 1400 °C without protective layers on GaN surfaces, n-type MOS capacitors with SiO₂ gate dielectrics formed on non-ion-implanted regions exhibited well-behaved capacitance–voltage characteristics with negligible hysteresis and frequency dispersion, indicating distinct impact of UHPA in suppressing surface degradation during high-temperature annealing. Efficient activation of the implanted Mg dopants and reasonable hole accumulation at the SiO₂/GaN interfaces were also achieved for p-type capacitors by UHPA, but the fabricated inversion- and accumulation-type p-channel GaN MOSFETs were hardly turned on. The findings reveal extremely low hole mobility at GaN MOS interfaces and suggest an intrinsic obstacle for the development of GaN-based MOS devices.