GMIP, a member of the RhoGAP family, plays a critical role in cytoskeletal remodelling, cell migration and immune modulation. Its aberrant expression in cancers suggests a pivotal role in tumour progression. GMIP expression was assessed using transcriptomic datasets from GDC and UCSC XENA, and protein distribution across tissues via HPA and GeneMANIA. The TISCH database identified primary GMIP-expressing cell types in the tumour microenvironment. Univariate Cox regression assessed GMIP's prognostic potential, while cBioPortal and GSCA explored genomic alterations. TIMER 2.0 was used to investigate immune cell infiltration and GMIP's role in immune regulation. GSEA and GSVA unveiled GMIP-related biological pathways, and molecular docking with CellMiner identified potential drug interactions. In vitro assays confirmed GMIP's functional relevance in breast cancer. GMIP exhibits differential expression across multiple cancer types, demonstrating significant prognostic implications. Its expression is inversely correlated with CNV and methylation in several cancers. GMIP is closely linked to immunotherapy biomarkers and immune suppression, influencing therapeutic responses. Functional studies suggest that GMIP inhibition reduces cancer cell proliferation and migration. GMIP is identified as a promising oncological biomarker, particularly in breast cancer, with potential therapeutic implications. GMIP's therapeutic potential is especially pronounced in BRCA-mutated tumours, underscoring its relevance for novel anticancer interventions.