The III-V compound semiconductor, GaN, has become an excellent semiconductor material for developing the high-frequency and high-power electronic devices because of its excellent characteristics, including large band width, high thermal conductivity and fast electron saturation rate, and has received extensive attention in recent years. However, the decomposition temperature of GaN is lower than the melting temperature, some of its fundamental properties, such as melting temperature and high temperature phase transition pressure, are still unclear, and so, now the investigation of fundamental properties dominates the whole process of this material from development to mature applications. In the present work, the classical molecular dynamics simulations combined with the first-principles calculations and lattice dynamics methods are adopted to predict the phase diagrams of GaN with wurtzite and rocksalt structures in a pressure range of 0–80 GPa. The phase transition pressures, 44.3 GPa and 45.9 GPa, obtained from the first-principles calculations and molecular dynamics simulations from wurtzite to rocksalt structure in GaN at zero temperature, are in agreement with the available experimental results (Sadovyi B, et al. 2020 Phys. Rev. B 102 235109). The melting temperature at 0 GPa is 2295 K obtained by extrapolating the GaN melting curve of the wurtzite structure. With the pressure increasing to 33.3 GPa, the melting curve of wurtzite structure in GaN intersects with the melting curve of rocksalt structure, and the melting temperatures of both structures increase with pressure increasing. It is found that GaN may have a superionic phase and the superionic phase transition occurs in the wurtzite structure at pressures greater than 2.0 GPa and temperatures above 2550 K, whereas the rocksalt structure undergoes a superionic phase transition at pressures and temperatures higher than 33.1 GPa and 4182 K, respectively, and both of the phase transition temperatures increase with pressure increasing. The slope of the phase boundary line of GaN is positive at high temperatures and gradually changes into a curve with a negative slope as the temperature decreases.