<p>Gallium selenide (GaSe), with its layered crystal structure and attractive optoelectronic characteristics, has emerged as a compelling material for next-generation devices. In this study, comprehensive spectroscopic ellipsometry analyses were performed to investigate the linear and nonlinear optical properties of GaSe over a wide photon energy range. The dielectric function parameters were modeled using the Wemple–DiDomenico single-oscillator approach, enabling accurate extraction of refractive index dispersion and related optical constants. Furthermore, the optical and electrical conductivity were derived and analyzed, revealing distinct features that reflect the interband transitions and charge transport behavior of the material. Nonlinear optical parameters, including the first- and third-order susceptibility as well as the nonlinear refractive index, were evaluated based on the linear dispersion data. The results confirm that GaSe exhibits strong optical responses in both the linear and nonlinear regimes, highlighting its potential for integration into optoelectronic and nonlinear frequency conversion devices.</p>