It is gamma-aminobutyric acid. It is an inhibitory neurotransmitter that some people use to elevate growth hormone production and aid sleep. You need to take multi gram doses orally for it to work though, because it doesn't efficiently cross the blood to brain barrier.
GABA does cross the blood brain barrier, but inefficiently. That is why you need multi-gram doses. Also just because GABA does not cross the blood brain barrier efficiently does not mean that it has no somatic effects. It can react in an inhibitory way on the peripheral nervous system.