The aim of this study was to investigate the diffusion of red-stained experimental GM primer and blue-stained dentin bonding agent into EDTA-conditioned dentin and the effect of GM priming on contraction gap formation. After GM priming at 0, 35, 70, or 100 vol%, marginal adaptation was evaluated by measuring the wall-to-wall contraction gap width of a light-activated resin composite (Palfique Estelite, Tokuyama Dental, Japan) mediated with a commercial dentin bonding agent (Clearfil Photo Bond, Kuraray Medical, Japan). Contraction gap formation was completely prevented when treated with 35 vol% GM primer. Moreover, after treatment with 35 vol% GM, the red and blue dyes of stained GM primer and dentin bonding agent diffused up to a depth of 40 µm into the dentin surface. The diffusion of primer and bonding agent into EDTA-conditioned dentin was essential for bonding efficacy although SEM observation revealed an absence of hybrid layer formation.