We present a depolarization-resolved spectroscopic ellipsometry investigation of epitaxial Ge1−xSnx alloys grown on GaAs substrates via molecular beam epitaxy. The study explores how Sn composition (0–6%), substrate orientation ((100) with varying offcut angles, (110), and (111)A), and lattice-matching configuration (Ge0.94Sn0.06/In0.12Al0.88As) influence the optical properties of GeSn. Accounting for depolarization significantly improves the resolution of extracted optical constants and critical-point (CP) features in GeSn heterostructures. Using second-derivative CP analysis, we extract E0, E0 +∆0, E1, and E1 +∆1 transition energies. Increasing Sn content produces a systematic redshift of all CP energies and an increase in n, k, ε1, and ε2. The extracted ∆Eg[Γ] value reduces with Sn incorporation. Both substrate orientation and offcut angle introduce measurable changes in ∆Eg[Γ]. These variations arise from four attributes: alloy bandgap reduction, compressive strain–induced bandgap increase, offcut-dependent growth kinetics, and the GeSn:Ge thickness ratio. A lattice-matched Ge0.94Sn0.06 epilayer grown on an In0.12Al0.88As-capped linearly graded InxAl1−xAs buffer, exhibits a higher refractive index across the entire measured spectral range, together with significantly larger absorption coefficients and dielectric-function amplitudes in the 2–4.5 eV region. These findings demonstrate that lattice-matched Ge0.94Sn0.06/In0.12Al0.88As yields an optical advantage and demonstrates that depolarization-resolved ellipsometry provides a more reliable measurement framework for optical properties. This provides a quantitative reference for the design of high-performance GeSn-based photonic and optoelectronic devices.