This study investigates gas flooding for enhanced oil recovery (EOR) in deep, high water-cut reservoirs under high-pressure and high-temperature conditions (50 MPa, 112 ℃). Experimental results show that CO₂ and natural gas yield similar oil swelling effects, with swelling factors of 1.41 and 1.37, respectively, at 60 mol% gas concentration. Core-scale flooding tests recovered an additional 40.7%, 36.1%, and 9.8% oil after water flooding using CO₂, natural gas, and N₂, respectively. Field-scale simulations revealed that gas flooding improves oil production in the upper layers due to buoyancy effects, overcoming limitations of water flooding. However, reservoir heterogeneity reduced EOR efficiency. Final recoveries increased by 21.3%, 18.5%, and 6.9% for CO₂, natural gas, and N₂ flooding, respectively. Natural gas offered faster oil production compared to CO₂, with comparable ultimate recovery. This study provides practical insights for designing gas flooding processes in deep reservoirs.