These are different (and similar) substances. One (GABA) is a neurotransmitter in the central nervous system that controls nervous system 'excitability'. The other (Gabapentin) is a drug that is an 'analogue' of GABA; ie it was manufactured to mimic the effects of GABA but it does not appear to effect the same receptors in the brain. It is, however, used to reduce nervous system excitability in epilepsy and in disorders with neuropathic pain such as fibromyalgia and chronic fatigue syndrome. These two substances, in the end, are similar and different.