Rather than taking a amino acid capsule/supplement, I'd recommend using a quality Whey powder. You can make smoothies out of Whey and add fruit or flax oil or lecithin as you choose. Protein is a macro nutrient (food), and needs to be consumed in larger quantities than capsules provide. Vitamins and minerals are classed as micronutrients, and can be delivered in tablet and capsule form.
If you don't want to use Whey, then a soy isolate protein is available also.
If you don't want to invest in large containers of whey protein, I've found WalMart carries smaller jars of the various types, so you can experiment that way with less cost.

Here is a very good article on GABA... note that increasing serotonin will increase GABA.

http://www.denvernaturopathic.com/news/GABA.html