It doesn't significantly cross the blood-brain barrier, so is unlikely to produce noticeable mental effects. I'm not sure if the GABA might convert to BBB-permeable chemicals outside the brain; it might explain why some people report effects.
Best bet for getting GABA-like compounds into the brain is with stuff like progabide, picamilon and GABOB. Easily found on the internet.
I used to eat ramen noodles a lot and sometimes it would make me kind of jittery. They have a lot of msg in them - monosodium glutamate. And glutamate is the primary excitatory neurotransmitter, and some of it would be converted to GABA, the primary inhibitory neurotransmitter.
Apparently it's restricted from crossing freely into the brain, but it's carried across by transport proteins. And some older parts of the brain allow it free passage. So if you took a lot of glutamate, it would probably make you more jittery than calm.
Although the BBB helps protect most of the brain from changes in circulating plasma L-glutamate, there are a few brain areas that do not contain a BBB and do allow rapid L-glutamate uptake from the circulation. These are known collectively as "the circumventricular organs" and include the median eminence, area postrema, subfornical organ, subcommissural organ, pineal gland, neurohypophysis and organum vasculosum of the lamina terminalis. Brain uptake rates for small solutes in these areas exceed those of normal brain by 10- to 1000-fold. Once within brain extracellular fluid, solutes can move into adjacent brain areas via intercellular diffusion or via flow along the Virchow-Robin spaces. Such movement has been documented for glutamate and aspartate in animals after high dose amino acid administration. The net result is that certain areas of the brain are vulnerable to acute fluctuations in plasma glutamate concentration of large magnitude as a result of "flooding" from the circumventricular organs.
Even if GABA itself occasionally finds its way into the brain, the sheer amount you'd have in circulation from such a dose makes it not such a good idea. People report chest tightness and paraesthesia (tingling) due to peripheral GABA receptors, which don't sound good to me.