Optical properties of intermetallic compounds GdNi5 - x Cu x (x = 0, 0.5, 1, 1.5, and 2) have been studied in a spectral range from 0.22 to 15 μm using the ellipsometry method. The substitution of copper for nickel has been found to lead to local changes in the interband optical conductivity spectra. A new quantum-absorption band, whose intensity increases substantially with increasing copper content, has been found at 3-4 eV. The relaxation and plasma frequencies of conduction electrons, which were calculated using data on the optical parameters, also depend on the concentration. Self-consistent calculations of the electronic structure of the intermetallic binary GdNi5 compound have been performed using the LSDA + U method. The density of electronic states for two spin projections and the frequency dependence of the interband optical conductivity of the compound have been calculated.