We report a high-performance field-effect transistor (FET) and phototransistor based on back-gated multilayer GaTe nanosheets. Through both electrical transport measurements at variable temperatures and first-principles calculations, we find Ga ion vacancy is the critical factor that causes high off-state current, low on/off ratio, and large hysteresis of GaTe FET at room temperature. By suppressing thermally activated Ga vacancy defects at liquid nitrogen temperature, a GaTe nanosheet FET with on/off ratio of ∼10(5), off-state current of ∼10(-12) A, and negligible gate hysteresis is successfully demonstrated. Furthermore, a GaTe phototransistor with high photogain above 2000 and high responsivity over 800 AW(-1) is achieved, as well. Our findings are of scientific importance to understand the physical nature of intrinsic GaTe transistor performance degradation and also technical significance to unlock the hurdle for practical applications of GaTe transistors in the future.