Catalyzes the oxidation 4-aminobutanal (gamma-aminobutyraldehyde) to 4-aminobutanoate (gamma-aminobutyrate or GABA). This is the second step in one of two pathways for putrescine degradation, where putrescine is converted into 4-aminobutanoate via 4-aminobutanal. Also functions as a 5-aminopentanal dehydrogenase in a a L-lysine degradation pathway to succinate that proceeds via cadaverine, glutarate and L-2-hydroxyglutarate. 4-aminobutanal + H2O + NAD(+) = 4-aminobutanoate + 2 H(+) + NADH 5-aminopentanal + H2O + NAD(+) = 5-aminopentanoate + 2 H(+) + NADH Amine and polyamine degradation; putrescine degradation; 4-aminobutanoate from 4-aminobutanal: step 1/1. Amino-acid degradation. Homotetramer. 4-aminobutanal can spontaneously cyclize to 1-pyrroline, and 5-aminopentanal to 1-piperideine. Belongs to the aldehyde dehydrogenase family. Gamma-aminobutyraldehyde dehydrogenase subfamily.