GABA affects cortisol production in the body (At least. Can't remember just how at the moment and am too lazy to google it), so doesn't need to pass the blood/brain barrier to have an effect on the brain.
Woah, it can have a strong effect! Even at one dose of 250mg early in day, reliably it makes me sooo sleepy, sleep way too long later that night, and gives me unpleasant dreams.
It reduces cortisol, but that doesn't mean it can pass the blood/brain barrier. The two have nothing to do with each other.