OBJECTIVE: Gastric cancer (GC) is a leading cause of cancer mortality, with being the second most frequently mutated driver gene in GC. We sought to decipher -specific GC regulatory networks and examine therapeutic vulnerabilities arising from loss. DESIGN: Genomic profiling of GC patients including a Singapore cohort (>200 patients) was performed to derive mutational signatures of inactivation across molecular subtypes. Single-cell transcriptomic profiles of -mutated GCs were analysed to examine tumour microenvironmental changes arising from loss. Genome-wide ARID1A binding and chromatin profiles (H3K27ac, H3K4me3, H3K4me1, ATAC-seq) were generated to identify gastric-specific epigenetic landscapes regulated by ARID1A. Distinct cancer hallmarks of -mutated GCs were converged at the genomic, single-cell and epigenomic level, and targeted by pharmacological inhibition. RESULTS: We observed prevalent inactivation across GC molecular subtypes, with distinct mutational signatures and linked to a NFKB-driven proinflammatory tumour microenvironment. -depletion caused loss of H3K27ac activation signals at -occupied distal enhancers, but unexpectedly gain of H3K27ac at ARID1A-occupied promoters in genes such as and . Promoter activation in -mutated GCs was associated with enhanced gene expression, increased BRD4 binding, and reduced HDAC1 and CTCF occupancy. Combined targeting of promoter activation and tumour inflammation via bromodomain and NFKB inhibitors confirmed therapeutic synergy specific to -genomic status. CONCLUSION: Our results suggest a therapeutic strategy for -mutated GCs targeting both tumour-intrinsic (BRD4-assocatiated promoter activation) and extrinsic (NFKB immunomodulation) cancer phenotypes.