The function of ENSG00000125875 (TBC1D20, TBC1 domain family member 20) is as follows. GTPase-activating protein (GAP) specific for Rab1 and Rab2 small GTPase families for which it can accelerate the intrinsic GTP hydrolysis rate by more than five orders of magnitude (PubMed:23236136). Also shows GAP activity for RAB18 GTPase (PubMed:26063829). Promotes RAB18 dissociation from the endoplasmic reticulum (ER) membrane into the cytosol, probably through stimulating RAB18 GTP-hydrolysis (PubMed:26063829). Involved in maintaining endoplasmic reticulum structure (PubMed:24891604). {ECO:0000269|PubMed:23236136, ECO:0000269|PubMed:24891604, ECO:0000269|PubMed:26063829}.