Plays a role in T lymphocyte development and the optimal generation of CD4/CD8 double-positive thymocytes (PubMed:16509771). Inhibitor of GSK3A. May act 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 (PubMed:29382851). Required for the survival of bone marrow hematopoietic stem cells, as well as of peripheral T cells, natural killer (NK) and NK T-cell development and the maintenance of normal liver function (PubMed:18796632, PubMed:21502331). May promote the survival of mature T lymphocytes upon cytokine withdrawal (PubMed:16509771). May regulate Ca(2+) homeostasis by modulating lysosomal Ca(2+) stores, preventing its accumulation in the absence of T cell activation (By similarity). May play a role in mitochondrial DNA segregation in hematopoietic tissues (PubMed:25808953). Is a regulator of liver endothelial cell homeostasis (PubMed:33956074). Interacts with BAD, BAK1, BAX, BCL2, BCL2L1/Bcl-xL and BCL2L11/BimEL (PubMed:16509771, PubMed:21502331). The interaction with BAX is increased, when cells initiate apoptosis upon IL2 withdrawal (PubMed:16509771). 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 (PubMed:21502331). May interact (via N-terminus) with microtubules (By similarity). Expressed in thymus (in thymocytes), spleen (in splenocytes), lymph node and lung (PubMed:16509771). Highly expressed in T lymphocytes (PubMed:16509771, PubMed:21502331). Expressed in B cells and in distinct lineages of hematopoietic bone marrow cells, including natural killer, B, T, myeloid and erythroid lineages (PubMed:21502331). Expressed in liver endothelial cells (PubMed:33956074). Up-regulated upon the maturation of CD4/CD8 double-positive to CD4 single-positive thymocytes. Knockout mice are born at the expected Mendelian rate, with a normal sex ratio, but have a median survival of only 15 weeks. They exhibit chronic hepatic hematopoiesis and, in later stages, show pronounced hepatocyte apoptosis, leading to lethal liver failure. Loss of GIMAP5 function impairs peripheral T-cell survival, imposes a complete block of natural killer (NK) and NK T-cell development (PubMed:18796632). Mutant mice show progressive multilineage failure of bone marrow and hematopoiesis. Compared with that of wild-type counterparts, the bone marrow contains more hematopoietic stem cells, but fewer lineage-committed hematopoietic progenitors (PubMed:21502331). Belongs to the TRAFAC class TrmE-Era-EngA-EngB-Septin-like GTPase superfamily. AIG1/Toc34/Toc159-like paraseptin GTPase family. IAN subfamily.