Abstract: Gap-graded soils are widely present in complex structured ground, where their mechanical behavior is jointly controlled by the proportions of coarse and fine particles, initial density, and particle shape. Traditional state variables often fail to capture the effective contribution of fine particles to the force-chain network. In this study, realistic particle shapes are reconstructed using a multisphere clump approach, and discrete element simulations are conducted to systematically examine the effects of particle shape, fine content, and density on stress transmission and small-strain stiffness. Results show that non-spherical particles generate a denser packing structure, enabling fine particles to engage in load transfer at lower contents; however, their stress-carrying efficiency remains consistently lower than their volumetric proportion. Building on these observations, a refined void ratio incorporating stress efficiency is proposed. This parameter simultaneously reflects the stress-transfer disparity between particle groups and the effective load-bearing skeleton, and significantly improves the prediction of small-strain stiffness in non-spherical particle systems. The findings highlight the critical role of particle-scale stress heterogeneity in controlling small-strain stiffness and provide a new state-variable framework for engineering analysis of gap-graded soils.