Required for mitochondrial integrity and T-cell survival. May contribute to T-cell quiescence. Plays a role in T lymphocyte development and the optimal generation of CD4/CD8 double-positive thymocytes (By similarity). Inhibitor of GSK3A, possibly by sequestering GSK3A in cytoplasmic vesicles and impairing its translocation to the nucleus. Consequently, impairs GSK3A-dependent transcriptional program and regulation of the DNA damage response occurring during T cells proliferation (By similarity). Required for the survival of peripheral T cells, natural killer (NK) and NK T-cell development and the maintenance of normal liver function (By similarity). Promotes the survival of quiescent T-cells (PubMed:15307172). May regulate Ca(2+) homeostasis by modulating lysosomal Ca(2+) stores, preventing its accumulation in the absence of T cell activation (PubMed:28223986). May play a role in mitochondrial DNA segregation in hematopoietic tissues (By similarity). Is a regulator of liver endothelial cell homeostasis (By similarity). Interacts with BAD, BAK1, BAX, BCL2, BCL2L1/Bcl-xL and BCL2L11/BimEL (By similarity). The interaction with BAX is increased, when cells initiate apoptosis upon IL2 withdrawal (By similarity). Forms a complex with BCL2L1 or MCL1 and HSPA8/HSC70; the interaction between HSPA8 and BCL2L1 or MCL1 is impaired in the absence of GIMAP5 (By similarity). May interact (via N-terminus) with microtubules (Probable). Primarily expressed in spleen, heart, lung and intestine and, at lower levels, in kidney, stomach and muscle (PubMed:12031988). Expressed in thymus and lymph nodes (at protein level) (PubMed:12031988, PubMed:12930893, PubMed:21487483). In the spleen, expressed in periarteriolar lymphatic sheets (PubMed:12031988). Isoform 2: Expressed at higher levels in T lymphocytes compared to isoform 1 (PubMed:28223986). Down-regulated during T lymphocyte activation. Diabetes-prone biobreeding (DP-BB) rats have a frameshift mutation in Gimap5, which results in a truncated protein in which the C-terminal 215 amino acids, including the membrane anchor, are replaced by 19 other amino acids. These animals exhibit life-long T-cell lymphopenia, including lack of regulatory T cells, and spontaneously develop insulin-dependent diabetes resembling human type 1 diabetes. Belongs to the TRAFAC class TrmE-Era-EngA-EngB-Septin-like GTPase superfamily. AIG1/Toc34/Toc159-like paraseptin GTPase family. IAN subfamily.