Deaminates gamma-aminobutyric acid (GABA) to succinate-semialdehyde, which in turn is converted to succinate by the succinate semialdehyde dehydrogenase (By similarity). Required for the degradation of GABA, which is important for utilization of GABA as nitrogen source. 2-oxoglutarate + 4-aminobutanoate = L-glutamate + succinate semialdehyde Homodimer. Belongs to the class-III pyridoxal-phosphate-dependent aminotransferase family.