Room-temperature dielectric function ε = ε₁ + iε₂ spectra of magnetoelectric Ga_2−xFe_xO₃ (x = 0.9, 1.0, and 1.4) thin films are determined by spectroscopic ellipsometry (SE) as a function of Fe concentration x. The SE data are analysed by a multilayer model with a series of Tauc-Lorentz oscillators. While the threshold energies slightly decrease as x increases, the oscillator strength shows a strong composition-dependence for the major optical structure at ∼3.5 eV. The experimental data are compared to the ε spectra obtained by density functional theory (DFT) calculations. Even though the overall shape of ε spectra is consistent, the experimental data and calculated spectra show a clear discrepancy in the oscillator's strength ratio of the two optical structures at ∼3.5 and ∼6.0 eV. The DFT calculations suggest that a significant disordering in the cationic (Ga and Fe) sites in Ga_2−xFe_xO₃ is present in thin films, which influences their optical properties. This work demonstrates a successful application of optical characterization for determining the cationic sites occupation in thin films, which in turn improves our understanding of Physics and Chemistry in Ga_2−xFe_xO₃ thin films and paves a pathway to the development of new multifunctional devices.