Required for the degradation of gamma-aminobutyric acid (GABA), which is important for utilization of GABA as nitrogen source. Deaminates GABA to succinate-semialdehyde, which in turn is converted to succinate by the succinate semialdehyde dehydrogenase. Cannot transaminate beta-alanine (BAL). 2-oxoglutarate + 4-aminobutanoate = L-glutamate + succinate semialdehyde Homodimer. Belongs to the class-III pyridoxal-phosphate-dependent aminotransferase family.