Deaminates gamma-aminobutyric acid (GABA) to succinate-semialdehyde, which in turn is converted to succinate by the succinate semialdehyde dehydrogenase (By similarity). Not required for the utilization of GABA as nitrogen source. 2-oxoglutarate + 4-aminobutanoate = L-glutamate + succinate semialdehyde Homodimer. Up-regulated by GABA and beta-alanine. Is not subject to nitrogen catabolite repression. Belongs to the class-III pyridoxal-phosphate-dependent aminotransferase family.