The absorption and emission spectra of Tm3+ ions in gadolinium gallium garnet is reported in order to identify the Stark levels involved in pumped and laser emission processes of this system. The analysis of the laser-excited luminescence from Tm3+ multiplets 1D2, 1G4, 3H4 to the ground manifold 3H6 confirms the assignments of Stark levels, corresponding to D2 symmetry, obtained from absorption and provides additional information. Due to the complex multisite structure, the site-selective excitation and emission data have been also used. A parametrized crystal-field calculation, including nine parameters for D2 symmetry, fits the energy-level diagram satisfactorily.