To address the problems of sand boiling and structural deformation failure induced by internal erosion in gap-graded soils under seepage action, erosion tests under different hydraulic gradients were conducted on gap-graded soil specimens with fine particle contents of 20%, 30%, and 40% using an internal seepage erosion apparatus. Based on the discrete element method, the evolution processes of particle loss, contact force chains, and porosity were analyzed, and the mesoscopic erosion characteristics of gap-graded soils were explored. The results show that specimens with higher fine particle contents exhibit higher erosion rates and more intense erosion processes. Under low hydraulic gradients, seepage-induced compaction occurs, resulting in a decrease in porosity, and the variation of porosity exhibits spatial heterogeneity. The fine particle content affects the contact modes among particles within the soil and serves as a triggering factor for particle loss. The combined effects of contact force chain transformation and particle loss lead to erosion-induced failure of the soil.