Gastric cancer is marked by profound molecular and microenvironmental heterogeneity that limits therapeutic progress. Here, we present a 15-layer multi-omics atlas that integrates genomics, epigenomics, transcriptomics, proteomics, multiple post-translational modifications (PTMs), protein-protein interactions, metabolomics, and microbiome profiles from 159 primary gastric adenocarcinomas and 30 matched normal adjacent tissues. Using cell-state deconvolution, we define tumor ecotypes that refine genomic and histological subtypes by capturing distinct tumor microenvironment architectures linked to clinical outcomes and potential associations with immunotherapy response. Multi-omics integration prioritizes genomic and epigenomic aberrations and their associated vulnerabilities; defines ecotype-specific transcriptional programs, signaling pathways, PTMs, protein interaction networks, and metabolic regulation; and identifies microbiome features linked to ecotypes and resistance pathways. We further prioritize ecotype-, genomic subtype-, and cell type-specific targetable proteins using proteomic and PTM analyses within a tumor microenvironment context. This comprehensive atlas provides a systems-level blueprint for decoding gastric cancer heterogeneity and advancing precision oncology.