In this work, the GW63K (Mg-6.54Gd-3.93Y-0.41Zr, wt.%) alloy wire was utilized as the feedstock material and the thin-walled component was fabricated using wire-arc additive manufacturing technology (WAAM). The microstructural evolution during deposition and subsequent heat treatment was explained through multi-scale microstructural characterization techniques, and the impact of heat treatment on the strength-ductility synergy of the deposited alloy was systematically compared. The results showed that the microstructure of the deposited sample was mainly composed of fine equiaxed α-Mg grains and Mg₂₄(Gd,Y)₅ phase. The optimized solution heat treatment (450 °C × 2 h) had little effect on the grain size, but can effectively reduce the Mg₂₄(Gd,Y)₅ eutectic phase on the grain boundary, resulting in a significant increase in elongation from 13.7% to 26.6%. After peak-aging treatment, the strength of the GW63K alloy increased to 370 MPa, which was significantly higher than the as-built state (267 MPa). The superior strength in this study is attributed to the refinement strengthening imparted by the fine microstructure inherited in the as-built GW63K alloy, as well as the precipitation strengthening due to the formation of dense β' precipitates with a pronounced plate-like aspect ratio.