The synergistic effect of low Gd+Mn additions on the evolution of microstructure and mechanical properties of Mg-xGd-0.8Mn alloy was investigated. Gd addition shows a strong grain refinement effect on the extruded Mg-xGd-0.8Mn alloy, and leads to a continuous decrease in the area fraction of basal texture grains and the corresponding maximum density of texture components. However, the maximum density of the basal texture components grows abruptly as Gd content increases to 6 wt.%. When the Gd content is below 6 wt.%, the asymmetry of the tensile and compressive yield of the alloy is negatively correlated to the Gd content due to grain refinement and texture weakening effects. Besides, the contribution of grain refinement to higher alloy yield strength is more significant than that of grain orientation. Compared with the extruded Mg-xGd alloy, the extruded Mg-xGd-0.8Mn alloy shows a lower limit composition point that corresponds to solid solution strengthening and plasticizing effect (2 wt.% and 4 wt.%). Finally, the trend of basal slip and prismatic slip resistance variations of the extruded Mg-xGd-0.8Mn alloys was predicted.