In this work, we report on investigations of structure, spectroscopic properties and laser performances of, what we believe to be, a novel Er:YGGAG laser crystal. High crystalline quality is proved by an FWHW of XRC of 0.019°. Thermal conductivity of a 30 at.% Er:YGGAG crystal is determined as 4.98 W/(m·K). The refractive index is measured in the range of 400 to 1000 nm and fitting with Sellmeier equation is done. A broad fluorescence emission band is located at 2786∼2819 nm, suggesting that this crystal is favorable to realize tunable and ultrafast laser. Under the pump at 969 nm with a fiber-coupled diode laser, at 400 Hz repetition rate and 600 µs pulse duration, the 30 at.% Er:YGGAG delivered maximum average power of 506 mW with overall optical-to-optical efficiency of 12.4% and slope efficiency of 16.9%. The laser beam quality was characterized by M2 factors of 1.53 and 1.39 in horizontal and vertical directions, respectively.