Stimulated-emission cross-section spectra are determined for the ~1.5 μm and 3 μm transitions of Er3+ ions in YSGG crystal. For the 4I11/2→4I13/2 channel, the maximum stimulated-emission cross-section σSE is 0.43×10−20 cm2 at 2797.1 nm. For the 4I13/2→4I15/2 channel, σSE=1.20×10−20 cm2 at 1532.8 nm. Due to the reabsorption loss, laser operation is expected at ~1644 nm. Radiative lifetimes of all excited states of the Er3+ ion from 4I13/2 to 2H9/2 and probabilities of radiative transitions from these states are determined using the Judd–Ofelt theory. Radiative lifetimes of the 4I13/2 and 4I11/2 excited states for Er3+ ions in YSGG are 7.73 ms and 9.75 ms, respectively. Non-radiative decay is analyzed for lower excited-states of Er3+ ions in YSGG.