An Er3+:Yb3+:GdAl3(BO3)4 (GAB) crystal with Yb3+ and Er3+ atoms fraction of 20% and 1.1% was grown by the flux method. End-pumped by a fiber-coupling quasi-continuous wave (CW) diode laser at 0.97 μm, 1.5～1.6 μm laser with output power up to 1.75 W and slope efficiency near to 20% was realized from a 0.7 mm thick Er3+:Yb3+:GAB crystal in a hemispherical cavity with the output coupler transmission of 1.5%. The influences of the absorbed pump powers and output coupler transmissions on the laser spectra were also investigated. When the output coupler transmission was 1.5% and the absorbed pump power was increased, more longitudinal mode groups were observed and the main output laser power gradually shifts from the longitudinal mode group around 1.60 μm to around 1.55 μm. When the absorbed pump power was 13.6 W and output coupler transmission was increased, laser wavelength was shifted from 1.60 μm to 1.52 μm. The results show that the Er3+ and Yb3+ co-doped GAB crystal is an excellent gain medium for realizing 1.5～1.6 μm laser.