To understand the operating slip modes and inhomogeneous plastic deformation quantitatively and statistically for a high-performance cast Mg-10Gd-3Y-0.5Zr (wt.%, GW103) alloy during room-temperature uniaxial compression, detailed slip trace analysis and electron backscatter diffraction (EBSD) based misorientation analysis of this alloy in aged condition were carried out. After 2% plastic strain, according to relative frequency of identified slip traces, the active slip modes were basal áa？ slip (73.3%), followed by prismatic áa？ slip (15.8%), then second-order pyramidal ác+a？ slip (6.9%), and finally first-order pyramidal áa？ slip (4%). Although most of the active slip systems exhibited large Schmid factor (m) values (>0.3), it was worth noting that some hard-oriented (m<0.1) slip systems were also active. For most of the grain boundaries exhibiting extremely large geometrically necessary dislocation (GND) density, at least one of the following conditions was satisfied: large grain boundary misorientation angle (GBMA) and/or large deviation of m_max for particular slip mode between neighboring grains. The plastic heterogeneity of grains (magnitude/distribution of GND density) was independent of the visibility of slip traces. The grain orientation speared (GOS) and/or grain average GND density showed no obvious correlation with m_max (for particular slip mode).