Samples of the initial composition Mg1-xNixGa2O4 (0 <= x <= 1, step x = 0.1) synthesized by the gel combustion method and annealed at 1000 degrees C have been studied by X-ray powder diffraction and IR spectroscopy. For homogeneous samples of (Mg,Ni)Ga2O4 with a spinel structure, the gallium nonstoichiometry has been determined by inductively coupled plasma atomic emission spectrometry. Within the framework of the MgO-NiO-Ga2O3 system, the boundary of the homogeneity region of spinel (Mg,Ni)Ga2O4, which is in equilibrium with halite (Mg,Ni)O, has been outlined. An analysis of the diffuse reflectance spectra of (Mg,Ni)Ga2O4 in the range of 350-900 nm revealed intense absorption bands from Ni2+ in octahedral positions and showed an increase in their intensity with an increase in the nickel content in the spinel.