Triply periodic minimal surface (TPMS) lattices are promising lightweight impact absorbers, but high-fidelity finite element modeling is costly, and uniform designs can develop high stress peaks during densification. This study presents a voxel-based finite element framework and a topology–density dual-gradient (TDDG) TPMS absorber. A convergence study selected a voxel size of 0.33 mm, yielding a 1.11% relative-density error and converged mechanical responses. Compared with a converged C3D4 tetrahedral model, the C3D8R voxel model reduced wall-clock time from 4222 to 497 s. Following validation against quasi-static compression tests, P, G, and IWP topologies at 20%, 30%, and 40% relative densities were screened. IWP20 and P40 were assigned to the impact- and support-facing regions and connected by normalized sigmoid interpolation. Under a 125 J impact, simulations predicted that TDDG-IWP20-P40 reduced peak nominal impact stress by 32.2% and 24.6% relative to U-P30 and DG-P20-P40, respectively, while maintaining comparable SEA. Additional simulations at 62.5 and 160 J confirmed lower peak stress than U-P30, with the added benefit over density-only grading becoming more pronounced at higher impact energy. Progressive crushing and delayed densification demonstrate the potential of TDDG TPMS absorbers for impact-load mitigation and future aerospace buffer designs.