Gas-insulated systems (GIS/GIL) require epoxy spacers with superior dielectric reliability under HV operation; however, pure epoxy suffers from charge accumulation, surface flashover, and limited breakdown strength. This study explores Zinc–Aluminum Layered Double Hydroxide (ZnAl-LDH) nanofillers to overcome these limitations through a comprehensive experimental–simulation framework. Epoxy nanocomposites with 1–7 wt% ZnAl-LDH were fabricated and systematically tested for volume and surface resistivity, surface charge dynamics, and flashover behavior. Results revealed moderate improvement in volume resistivity, while surface resistivity rose by 52.6%, effectively suppressing leakage conduction. Surface potential decay tests showed 40% faster charge dissipation in the nanocomposite, attributed to increased shallow trap density, yielding a 9.3% rise in flashover voltage. ISPD trap analysis confirmed a shift of the shallow trap dominance region from 0.917 eV to 0.936 eV, clarifying the mechanism of accelerated de-trapping. Finally, GIS-scale simulations validated practical applicability, ZnAl-LDH incorporation reduced peak surface charge density by 40.7% and mitigated triple-junction field stress by 14.7% under combined electro-thermal and DC charging. This work provides the first integrated evidence that ZnAl-LDH/epoxy nanocomposites can suppress surface charging while bridging the gap between material-level optimization and spacer-level performance, establishing ZnAl-LDH/epoxy as a promising candidate for next-generation HVDC GIS/GIL insulation.