By density functional theory of quantum chemistry, the gas reaction mechanism in GaN HVPE growth is extensively studied, especially the reaction of GaCl with NH3, the elimination of H2 or Cl2 from GaCl3:NH3, and the formation of H radicals caused by the evaporation of liquid gallium and its subsequent reactions. By calculations of Gibbs free energies and energy barriers at different temperatures, the reaction probabilities are determined both thermodynamically and kinetically. The results show that Δ G >0 for the whole six possible reactions between GaCl and NH3, which indicates that those reactions are unfavorable. For the three reactions with elimination of H2 or Cl2 from GaCl3:NH3, the reactions are also unfavorable. By thermodynamic calculation of Ga phase change, when T >1200 K, the Ga vapor pressure rises rapidly. Thus, the gaseous Ga in gas phase reaction cannot be neglected. The gaseous Ga reacts with HCl to form GaCl and H radicals, and H radicals will finally result in the amino compounds GaClNH2 and GaCl(NH2)2, which are also unfavorable to further react with NH3. Thus, we conclude that, in addition to GaCl, GaClNH2 and GaCl(NH2)2 are also the most likely Ga-containing molecules in the HVPE reactor, which will provide both GaN surface growth and nanoparticles.