In this paper, the Fe/Si reduction efficiency, Gd/Mn alloying efficiency, and corrosion susceptibility of the Mg-xMn-4.0Gd alloy were investigated. The results reveal that adding Mn gives a more prominent Fe removal effect under the conventional smelting processes. Neither the amount of Gd addition nor excessive Mn addition is sufficient for the continued strengthening of the Fe removal effect. The alloying efficiency of Mn increased and then decreased as a result of adding Mn alone to mixtures of Mn and Gd elements, while the alloying efficiency of Gd elements gradually diminished. An appropriate amount of Mn addition is beneficial for efficiently improving the corrosion resistance of the alloy. The extruded Mg-0.8Mn-4.0Gd alloy has more outstanding corrosion resistance, with a CRWeighet Loss of 4.48 ± 0.33 mg/(cm2·d) and CRHydrogen Evolution of 2.01 ± 0.31 ml/(cm2·d). The corrosion susceptibility of the extruded Mg-xMn-4.0Gd alloy mainly hinges on the grain size, Gd solid solubility, and texture intensity of basal plane I(0002).