<p>This research uncovers the plastic characteristics and underlying mechanisms in an Mg–Gd–Y alloy under a wide range of stress triaxiality conditions using crystal plasticity modeling. The study covers uniaxial tension, uniaxial compression, plane strain tension, plane strain compression, and shear. Several mechanical experiments were conducted under various uniaxial loading conditions. The material parameters of the visco-plastic self-consistent equation with the predominant twin re-orientation (VPSC-PTR) model were optimally calibrated using a developed experiment-inverse engineering method based on the measured texture. An obvious tension-compression asymmetry is observed in terms of yield stress and hardening behavior for the Mg–Gd–Y alloy. The deformation behavior predicted by the VPSC-PTR model matches the experimental result with high accuracy. As the equivalent strain increases, the slip system becomes the dominant deformation mode. The basal pole of grain rotates perpendicular to the loading direction under tension, while it reorients under compression and shear conditions. This research contributes to understanding the structure–property relationship of the Mg–Gd–Y alloy and provides valuable insight for alloy design.</p>