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CuGaS2 is the most promising chalcopyrite host for intermediate-band thin-film solar cells. Standard Kohn-Sham density functional theory fails in describing the band structure of chalcopyrite materials, due to the strong underestimation of the band gap and the poor description of p-d hybridization, which makes it inadvisable to use this approach to study the states in the gap induced by doping. We used a state-of-the-art restricted self-consistent GW approach to determine the electronic states of CuGaS2: in the energy range of interest for optical absorption, the GW corrections shift the Kohn-Sham bands almost rigidly, as we proved through analysis of the effective masses, bandwidths, and relative position of the conduction energy valleys. Furthermore, starting from the GW quasiparticle bands, we calculated optical absorption spectra using different approximations. We show that the time-dependent density functional theory can be an efficient alternative to the solution of the Bethe-Salpeter equation when the exchange-correlation kernels derived from the Bethe-Salpeter equation are employed. This conclusion is important for further studies of optical properties of supercells including dopants.