[en] The spectral and laser properties of Gd₃Ga₅O₁₂:Nd³⁺ and Nd₃Ga₅O₁₂ crystals are investigated. Highly perfect crystals are grown by a sophisticated Czochralski technique. Absorption, luminescence and stimulated emission spectra are studied at 77 and 300 K. The luminescence lifetime of the metastable ⁴Fsub(3/2) state in crystals with high Nd content and the dependence of the spectroscopic quality parameter X on the luminescence intensity parameter Y are determined accurately, using new methods. Further inter-manifold and 'Stark' luminescence branching ratios, spontaneous transition probabilities, luminescence linewidths, and peak and effective transition cross sections are obtained. Stimulated emission is observed at 300 K in the cw regime due to the basic ⁴Fsub(3/2) → ⁴Isub(11/2) transition, and in the long pulse regime due to the additional ⁴Fsub(3/2) → ⁴Isub(13/2) transition. (author)