The process of obtaining a bulk mono-crystalline gallium-containing nitride from the gallium-containing feedstock in a supercritical ammonia-containing solvent characterized in that the feedstock is in the form of metallic gallium or mono-crystalline gallium-containing nitride, and the ammonia-containing solvent is in the form of ammonia with addition of mineralizer in the form of the Group I (IUPAC, 1989) elements and/or their mixtures, and/or their compounds, especially those containing nitrogen and/or hydrogen, there are two temperature zones in each step of the process, and the feedstock is placed in the dissolution zone and at least one mono-crystalline seed is deposited in the crystallization zone, and following the transition of the solvent to the supercritical state, the process comprises the first step of transition of metallic gallium to the solution at the first temperature, and then the second step of selective crystallization of gallium nitride on the feedstock in the form of mono-crystalline gallium-containing nitride, and then the third step of crystallization of the gallium nitride, through gradual dissolution of the feedstock and selective crystallization of gallium-containing nitride on at least one seed at the temperature higher than that of the dissolution of the feedstock, while all the vital components of the reaction system (including the feedstock, seeds and mineralizer) invariably remain within the system throughout the whole process, and subsequently bulk mono-crystalline gallium-containing nitride is obtained.