Significance We have identified a lethal subtype of gastric cancer (GC) that is characterized by high levels of clonal heterogeneity and TP53 (tumor protein P53) mutation. We have also uncovered key novel mutations in the targetable NRG1 (neuregulin-1) and ERBB4 (V-Erb-B2 avian erythroblastic leukemia viral oncogene homolog 4) ligand-receptor pair and identified BRCA2 (breast cancer 2, early onset) mutations as new genetic markers to predict better survival for GC. Our study represents a novel approach for GC personalized medicine and identified novel clinical actionable therapies for GC therapy.