We report a complete spectroscopic analysis of ${\mathrm{Nd}}^{3+}$ -doped ${\mathrm{Gd}}_3{\mathrm{Al}}_x{\mathrm{Ga}}_{5-x}{\mathrm{O}}_{12}$ garnet crystal (Nd:GAGG), including absorption and emission spectra at low and room temperatures, excitation spectra showing the multisite structure of the host crystal, and temperature dependence of the lifetime of the ${}^4{F}_{\frac{3}{2}}$ manifold. We also report the emission cross sections for the transitions from the ${}^4{F}_{\frac{3}{2}}$ manifold and a Judd–Ofelt analysis with the calculation of the spontaneous emission probabilities and the radiative branching ratios.