The gastric tumor microenvironment is dynamically shaped by the interactions between the local microbiota and the host immune system, although the functional integration of these elements remains incompletely understood. In this study, we characterized microbial diversity, immune cell composition, and immune-related gene expression profiles in samples of gastric adenocarcinoma (GAC) and adjacent peritumoral tissue (PTT), aiming to elucidate their functional organization. A total of 106 samples of 75 patients were analyzed using bulk RNA-Seq expression profiling, immune deconvolution, and bacterial taxonomic reconstruction. While alpha diversity remained preserved between GAC and PTT, distinct compositional differences emerged: GAC was enriched with Pseudomonadota, Enterobacteriaceae, and Escherichia, whereas PTT exhibited a predominance of Helicobacteraceae and Helicobacter. Immune deconvolution revealed an expansion of cancer-associated fibroblasts (CAFs) and mast cells in GAC, correlated with higher expression levels of TGFB1 and FOXP3, while neutrophils and B cells predominated in PTT. Integrated analysis demonstrated that GAC formed dense and cohesive networks connecting pro-inflammatory bacteria, activated immune cells, and inflammatory genes such as IL1B, CXCL8, and IFNG. In contrast, PTT exhibited dispersed networks and negative correlations, suggesting a less structured, tolerogenic environment. Our findings indicate that gastric cancer progression involves not only compositional shifts in microbiota and immune cells but also the active construction of functionally integrated inflammatory networks, providing new insights into potential therapeutic targets at the microbiome–immune interface.