Li7La3Zr2O12 (LLZO) garnet is one of the most promising Li-ion solid electrolytes for all-solid-state Li-ion batteries, owing to its high chemical stability against Li metal and relatively high Li-ion conductivity. In order to further enhance the conductivity of LLZO, Gd3+ is used to dope LLZO at the Zr4+ site, in order to enable excess Li to occupy the octahedral sites, which could facilitate Li-ion transport and increase ionic conductivity. In this study, Li7+xLa3Zr2-xGdxO12 (LLZGO, x = 0–0.5) solid electrolytes with cubic phase and high relative densities are prepared at 1220 °C using a solid-state reaction method. It is found that LLZGO garnets with x from 0.1 to 0.2 deliver higher conductivities than pristine LLZO. Among these, the Li7.2La3Zr1.8Gd0.2O12 (LLZG2O) sample achieves the highest room-temperature total conductivity of 2.3 × 10−4 S cm−1. X-ray diffraction characterization confirms that LLZG2O is chemically stable against Li metal at room temperature for half a month. Cyclic voltammetry shows that LLZG2O possesses good electrochemical stability and reversibility of Li dissolution and deposition reactions. Symmetrical cells using a LLZG2O solid electrolyte and two Li metal electrodes are cycled galvanostatically under various current densities. The results demonstrate that the cell can achieve excellent cycling stability and low overpotentials for the dissolution and deposition reactions of Li. The substitution of Gd3+ for Zr4+ is effective in improving the Li-ion conductivity of LLZO, and Li7+xLa3Zr2-xGdxO12 could be promising as solid electrolytes for high-performance all-solid-state Li-ion batteries.