The temperature effect of ammonium halogenides NH₄X (X = Cl, Br, I) on the phase stability of GaN synthesized under supercritical ammonothermal conditions in the temperature range 400–550 °C has been investigated. Self-nucleated hexagonal GaN (h-GaN), cubic GaN (c-GaN), and gallium oxide (Ga₂O₃) has been crystallized. The latter was formed from mineralizers containing a relatively high amount of oxygen. The tendency to form c-GaN increases from X = Cl over Br to I. Decrasing the temperature supports the formation of c-GaN. Single-phase h-GaN can be grown from X = Cl, Br at 550 °C. Solubility of GaN is discussed. The use of h-GaN substrate has a phase-stabilizing effect and lowers the stability range for overgrown h-GaN films. We show how the choice of precursor will have an impact on a and c lattice parameter of self-nucleated h-GaN.