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In this work we have performed a comparative analysis of optical absorption and micro-Raman spectra for series of Gd3Ga5O12 (GGG) single crystals irradiated by fast neutrons with fluences from 10(16) n/cm(2) to 10(18) n/cm(2). It was found that the optical absorption spectra of non-irradiated Czochralski grown GGG consist of the relatively narrow lines in the UV spectral range related to the 4f-4f transitions in Gd3+. Transitions from the S-6(7/2) ground state to the P-6, (6)J and D-6 states in a Gd3+ cation are clearly detected. For a GGG crystal containing Ca impurity ions, additional absorption band at 350 rim is observed, and it is tentatively ascribed to oxygen vacancies associated with Ca impurities. Several neutron-induced optical absorption bands, including ones related to F+ centres, were observed in fast neutron-irradiated Gd3Ga5O12 single crystals. Furthermore, clear evolution of the Raman spectra with neutron dose is also found. In particular, fine structure observed between 1350 and 1550 cm(-1) is completely disappeared at high fluences, while several significant changes and the emergence of new transitions have been ascertained in spectral range of 100-850 cm(-1).