Detail spectroscopy study of erbium-doped active media is crucial for understanding of the behavior of erbium-based lasers. In this paper, for the first time, detailed study of the concentration dependence of Er3+ in garnet-based active media is presented. Wide range of novel samples Er:GGAG (26 in total) with various Er3+ concentration (0.5–38 at%), were investigated for the first time. The complete spectroscopy (absorption and emission cross-section, Raman, mid- and far-infrared transmission spectra, decay times at 4I11∕2 and 4I13∕2) together with Judd-Ofelt analysis and laser characteristics were measured at room temperature. Results shows strong dependence of the first excited state (4I13∕2) on concentration, on the other hand, the 4I11∕2 level is completely independent of the erbium doping level. The Judd-Ofelt analysis show that erbium doping has minimal effect on the structural properties of Er:GGAG. Also it is shown that Er:GGAG laser has a slope efficiency of 15.3 % (almost half of the theoretical limit) and generated maximal output power of 350 mW.