The applications of 3D inorganic nanomaterials in environmental and agriculture monitoring have been exploited continuously; however, the utilization of semiconductor nanoclusters, especially for detecting warfare agents, has not been fully investigated yet. To fill this gap, the molecular modelling of novel inorganic semiconductor nanocluster Ga₁₂As₁₂ as a sensor for phosgene gas (highly toxic for living things and the environment) is accomplished employing benchmark DFT and TD-DFT investigations. Computational tools have been applied to explore different adsorption sites and the potential sensing capability of the Ga₁₂As₁₂ nanoclusters. The calculated adsorption energy (−21.34 ± 2.7 kcal mol⁻¹) for ten selected complexes, namely, Pgn–Cl@4m-ring (MS1), Pgn–Cl@6m-ring (MS2), Pgn–Cl@XY66 (MS3), Pgn–O@4m-ring (MS4), Pgn–O@XY66 (MS5), Pgn–O@XY64 (MS6), Pgn–O@Y (MS7), Pgn–planar@Y (MS8), Pgn–planar@X (MS9), and Pgn–planar@4m-ring (MS10), manifest the remarkable and excessive adsorption response of the studied nanoclusters. The explored molecular electronic properties, such as interaction distance (3.05 ± 0.5 Å), energy gap (∼2.17 eV), softness (∼0.46 eV), hardness (1.10 ± 0.01 eV), electrophilicity index (10.27 ± 0.45 eV), electrical conductivity (∼1.98 × 10⁹), and recovery time (∼3 × 10⁻¹² s⁻¹) values, ascertain the elevated reactivity and an imperishable sensitivity of the Ga₁₂As₁₂ nanocluster, particularly for its complex MS8. QTAIM analysis exhibits the presence of a strong electrostatic bond (positive ∇²ρ(r) values), electron delocalization (ELF < 0.5), and a strong chemical bond (because of high all-electron density values). In addition, NBO analysis explores the lone pair electron delocalization of phosgene to the nanocluster stabilized by intermolecular charge transfer (ICT) and different kinds of non-covalent interactions. Also, the green region existence expressed by NCI analysis (between the nanocluster and adsorbate) stipulate the energetic and dominant interactions. Furthermore, the UV-Vis, thermodynamic analysis, and density of state (DOS) demonstrate the maximum absorbance (562.11 nm) and least excitation energy (2.21 eV) by the complex MS8, the spontaneity of the interaction process, and the significant changes in HOMO and LUMO energies, respectively. Thus, the Ga₁₂As₁₂ nanocluster has proven to be a promising influential sensing material to monitor phosgene gas in the real world, and this study will emphasize the informative knowledge for experimental researchers to use Ga₁₂As₁₂ as a sensor for the warfare agent (phosgene).