Mg-xGd-0.6Zr (x = 2, 4, and 6% mass fraction) alloys were synthesized by semi-continuous casting process. The effects of gadolinium content and aging time on microstructures and mechanical properties of the Mg-xGd-0.6Zr alloys were investigated. The results show that the microstructures of the as-cast GKx (x = 2, 4, and 6%) alloys are typical grain structures and no Gd dendritic segregation. In as-cast Mg-6Gd-0.6Zr alloy, the second phases Mg(5.05)Gd, Mg(2)Gd, and Mg(3)Gd will form due to non-equilibrium solidification during the casting process, and these second phases will disappear after hot-extrusion. The residual compressive stress exists in alloys after extrusion and increases with increasing Gd content. The existence of residual compressive stress contributes to the tensile strength. The elongation of all extruded alloys is over 30%, and the ultimate and yield tensile strength of the Mg-6Gd-0.6Zr alloy are 237 and 168 MPa, respectively. After isothermal aging for 10 h, the strength of extruded Mg-6Gd-0.6Zr alloys increases slightly, however, the elongation of alloys rarely decreases. The fracture mechanism of all studied alloys is ductile fracture.