This study presents a micromechanical evaluation of the regimes delineating the behaviour of gap-graded granular assemblies, using discrete element simulations. Dense and loose bimodal assemblies of different fines content were prepared and subjected to drained triaxial compression until the critical state was reached. The regimes delineating the behaviour of the assemblies were evaluated, characterised and their significance discussed. While two regimes demarcated by the threshold fines content were identified based on the analysis of the macroscale characteristics of the assemblies, up to four regimes were identified based on the contributions of the particle size fractions and contact types to the total mean stress. Contrary to previous studies according to which fines control the mechanical behaviour of gap-graded assemblies from the threshold fines content, fcth, we found that the fines do not play a primary role in stress transmission until beyond a significantly larger fines content, feq (the equivalent fines content), which depends on density and stress state. Based on the correlation found between the critical state strength and the stress-based skeleton void ratio proposed in this study, we conclude that stressbased skeleton void ratio can be useful in understanding the mechanical response of gap-graded materials at the critical state.