Gastric digestion is crucial for protein breakdown. Magnetic resonance techniques have a great deal of potential but remain underexplored with regard to their application in the study of food digestion via MRI-markers, such as transverse (R2) and longitudinal (R1) relaxation rates. R2 has been used to monitor gastric digestion of whey protein gels, but only in a static in vitro model. It is essential to investigate whether relaxation rates can be valid measures of digestion under dynamic circumstances. We developed a novel MRI-compatible semi-dynamic gastric simulator (MR-GAS) that includes controlled gastric secretion, emptying and mixing at body temperature. PH and protein hydrolysis were measured during protein gel digestion in the MR-GAS. R2 and R1 of the supernatant were measured by time-domain nuclear magnetic resonance (TD-NMR). The stomach chamber of the MR-GAS was also scanned with MRI to measure R2 and R1. For TD-NMR, 99% of the variance in R2 and 96% of variance in R1 could be explained as a function of protein concentration and [H+]. For MRI, the explained variances were 99% for R2 and 60% for R1. From these analysis, the obtained equations enabled the prediction of protein concentration and pH by R2 and R1. The normalised root mean squared deviation of the predictions for protein concentration were 0.15 (NMR) and 0.18 (MRI), and for pH were 0.12 (NMR) and 0.29 (MRI). In conclusion, the MR-GAS model may be used in a clinical MRI to monitor gastric digestion under in vitro dynamic circumstances, by measuring R2 and R1. These results underscore the potential of MRI to monitor nutrients hydrolysis and pH changes in future in vivo studies.