Gastric cancer (GC) is one of the lethal cancer types as well as one of the heterogeneous cancer types. To elucidate GC molecular mechanisms, mutational co-occurrence analyses have been suggested. However, an association between mutational co-occurrences and known GC signaling contexts has yet to be identified. In this study, the known GC signaling contexts including cancer hallmarks (DNA repair, WNT signaling, Notch signaling) were inspected in terms of mutational co-occurrences, in particular, for a specific GC phenotype, microsatellite instability (MSI) statuses. By using correlations for measuring mutational co-occurrences of gene pairs belonging to the cancer hallmarks, we constructed mutational co-occurring networks for each MSI status. As a result, a certain MSI status, microsatellite stable (MSS), showed that JAG1 mutation was likely to co-occur for the genes belonging to WNT and Notch signalings. Our study may facilitate a new therapeutic strategy to regulate Notch signaling-based compound design in the GC MSS patients.