GABA is the most potent depressive neuroamine in human brains. It regulates many of the depressive and sedative actions in brain tissue and is critical for relaxation.
GABA is a highly regulated compound in vivo (in living), and is able to balance itself out in body tissues due to a myriad of factors.
Due to these regulation factors, GABA as a supplement does not exert many depressive effects on its own. The human body is too adept at regulation, and orally ingested GABA cannot alter human physiology to much of a degree.
GABA, however, is a target for many other compounds that can act vicariously (in a multitude of ways) to increase GABA levels, which ultimately causes depressive effects.