Mg–8Gd–2Y–Nd–0.3Zn (wt%) alloy was prepared by the high pressure die-cast technique. The microstructure, mechanical properties in the temperature range from room temperature to 573 K, and strengthening mechanism were investigated. It was confirmed that the Mg–Gd-based alloy with high Gd content exhibited outstanding die-cast character. The die-cast alloy was mainly composed of small cellular equiaxed dendrites and the matrix. The long lamellar-shaped stacking compound of Mg3X (X: Gd, Y, Nd, and Zn) and polygon-shaped precipitate of Mg5RE (RE: Gd, Y, and Nd) were mainly concentrated along the dendrite boundaries. Meanwhile, it was demonstrated that the Zn addition affects the formation of non-equilibrium precipitate Mg3X. The ultimate tensile strength, yield strength, and Young’s modulus were 302 MPa, 267 MPa, and 38 GPa at room temperature, respectively. The outstanding mechanical properties were mainly attributed to the small dendrite spacing, wide skin region, and some dispersed precipitates in the alloy formed by the high-pressure die-cast technique. Designing a novel die-cast Mg alloy with good heat resistance without Al element is a significant accomplishment.