<p>Mercury (Hg), a highly toxic heavy metal, poses a significant threat to ecosystems and human health due to its neurotoxicity, bioaccumulation, and environmental persistence in its neutral atomic form (Hg⁰). Therefore, developing two-dimensional materials (2D) with high adsorption efficiency and excellent electronic properties for efficient Hg⁰ removal is of great theoretical and practical value. In this study, we systematically investigated the structural stability, electronic properties, and Hg adsorption behavior of GaS monolayers doped with C, N, and O at the Ga and S sites, using density functional theory (DFT). We explored the mechanism by which doping regulates the photovoltaic properties of these materials. Thermodynamic and kinetic stability were confirmed through formation energy, phonon spectra, and molecular dynamics simulations. Six representative doping configurations were selected for further analysis. The adsorption energy calculations show that the doped systems exhibit stronger Hg⁰ adsorption than pristine GaS, with the Ga-site N-doped system showing the most negative adsorption energy and the best thermodynamic stability. Band structure and density of states (DOS) analysis reveal that doping and Hg adsorption can synergistically modify the electronic structure, narrowing the bandgap, introducing impurity states, and even inducing a semiconductor–metal transition, thereby enhancing electron mobility. Differential charge density and Bader charge analysis further indicate that doping type and site significantly influence the coupling strength at the Hg–GaS interface. In terms of optical properties, nonmetallic doping and Hg adsorption enhance GaS’s dielectric polarization, shift the absorption peak to longer wavelengths, increase the absorption coefficient, and reduce reflectance, indicating improved low-energy light response and photoconversion potential. In particular, C- and O-doped systems exhibit excellent light absorption in the visible–ultraviolet range. Nonmetallic doping not only improves GaS’s Hg adsorption capacity but also optimizes its electronic and optical properties. This study provides a theoretical foundation and strategies for designing high-performance Hg-adsorbing materials and dual-functional photoelectric–environmental 2D materials.</p>