Background: Protein digestion under gastric conditions is influenced by the activity of the enzyme pepsin. With a limited penetration depth, pepsin-induced protein hydrolysis mainly occurs at the surface of the gel particles. Understanding the diffusion of digestive enzymes within different casein gels is a key factor to better control protein digestion and absorption. Method: A confocal microscopy fluorescence recovery after photobleaching (FRAP) technique was applied in combination with quasi-elastic neutron scattering (QENS). While FRAP determines the kinetics at a millisecond scale, water mobility within the undigested and digested casein gels were characterised by QENS at a pico to nanosecond time scale for the first time. Results: Gels made in D2O hindered diffusion due to its homogeneous structure and hydrophobic nature. After two-hour of simulated gastric digestion, the rennet-induced gel (RG) prepared in H2O exhibited greater diffusion compared to the undigested RG gel, implying that the larger aggregates and pores formed during digestion allowed for increased diffusivity. Conclusions: Our results confirm that an increase in gel porosity is the main driver of pepsin diffusion from the gel surface to the interior and governs water population diffusion within the gel structure. This will be the first report to compare diffusion within D2O and H2O gels.