I'm happy that you found a puzzle piece. However I bet it is not a magic bullet for all of your problems.
We see this situation often in the heavy metal detox groups--including a close female friend with long history of PTSD and sleep problems, etc. HMs affect so many systems.
So while GABA is helping you to feel more calm, the reason(s) why you had those symptoms is complex.
Magnesium, B6 and glutamine are involved in production of GABA.
Adrenal problems might be causing you to make more adrenaline than you should relative to cortisol.
Low dose lithium supplements also can have a nice chilled out effect on people. Li is more often very low than any other essential element in the hair tests of detox group members.
Even just one or two milligrams of Li from Li orotate or aspartate can have a noticable effect on mood. I've been there.
Over time taking too much GABA might also contribute to constipation. However the magnesium that may be one of the underlying causes for your need for GABA helps against constipation.
My friend takes a combination of low dose Li, Bs, magnesium, glutamine, theanine, pregnenolone, GABA, etc. We will typically benefit from taking most of them several times per day for more stable mood, etc.