The growing demand for sustainable and energy-dense biofuels has intensified the search for low-cost, non-edible biomass feedstocks capable of producing diesel-range hydrocarbons with properties comparable to petroleum diesel. In this study, Gmelina arborea leaf biomass was valorised into green diesel (deoxygenated biodiesel) via a calcium hydroxide (Ca(OH)2)-catalysed thermal ethanolysis operated under mild conditions (40-60 °C, 40-60 min, 1-2% catalyst loading). A three-factor Box–Behnken Response Surface Methodology (RSM) was employed to systematically evaluate the individual, interaction, and quadratic effects of temperature, reaction time, and catalyst loading on diesel yield. Gas chromatography-mass spectrometry (GC-MS) analysis confirmed the formation of predominantly C10-C20 hydrocarbons, including n-alkanes, branched alkanes, and cycloalkanes, indicating effective deoxygenation and cracking of biomass-derived intermediates. The highest experimental yield was obtained at 60 °C, 50 min, and 2% Ca(OH)2, producing 720.35 mg/g of diesel-range hydrocarbons. ANOVA results demonstrated that temperature and catalyst loading were highly significant factors (p < 0.01), with a strong synergistic interaction between them, while reaction time exhibited a moderate but significant quadratic effect. The quadratic regression model was statistically significant (p < 0.0001) and highly predictive. Reproducibility assessment using replicated centre-point experiments showed a low coefficient of variation (1.22%), confirming excellent experimental precision and operational stability. The results demonstrate that mild alkaline-catalysed thermal hydrolysis provides an energy-efficient and statistically validated pathway for decentralised green diesel production from underutilised lignocellulosic leaf biomass, supporting sustainable bioenergy development and resource valorisation.