The influence of particle size ratio on the mechanical characteristics of dense and loose gap-graded granular assemblies with varying finer particle contents under drained triaxial compression is investigated using the discrete element method (DEM). The behaviour of assemblies designated as internally stable according to Kedzi’s filter internal instability criterion is compared with internally unstable assemblies. This study shows that the influence of size ratio on the mechanical characteristics of gap-graded assemblies depends on the finer particle content considered and is more pronounced within the underfilled assemblies than within the overfilled assemblies. The micromechanical characteristics such as the coordination number and the proportion of erodible finer fraction within the internally stable underfilled assemblies are distinct from the internally unstable assemblies, confirming Kedzi’s criterion. However, Kedzi’s internal instability criterion which is solely based on particle size distributions does not account for the evolution in particle contact formation and stress transmission occurring during shearing, and the consequent changes in the proportion of erodible finer fraction as observed in our DEM data. This limitation renders Kedzi’s criterion inadequate for assessing the internal instability of gap-graded granular assemblies subjected to shearing.