AlGaN/GaN HEMT is one of attractive candidates for next generation high power devices because of high carrier mobility in 2DEG channels and high breakdown voltage due high critical electric field. In order to apply the AlGaN/GaN HEMTs for power switching applications the normally off operation is required. Enhancement type behavior of GaN HEMT transistors is obtained by using p-type Mg-doped GaN gate structures. The optimized epitaxial designs enable threshold voltage close to +2V. In present work, it is shown that atomic hydrogen treatment of the Mg doped p-GaN before gate metal evaporation can increase the threshold voltage up to +3.5V. It can be caused by the hydrogen-induced dipole layer formation at the p-GaN semiconductor interface after atomic hydrogen treatment. Further increase of treatment time lead to reduce the threshold voltage by form neutral complexes. Neutral complexes reduce the Mg doping level of p-GaN layer. There was no visible parameters degradation after thermal annealing at T = 300 °C for t = 30 min in vacuum environment. It can be caused by the formation thermally stable Mg-H complexes in the p-GaN layer after hydrogenation.