According to this article: https://www.healthline.com/health/gamma-aminobutyric-acid#effectiveness
"Not much is known about the effectiveness of GABA supplements. In fact, experts don’t know how much GABA actually reaches the brain when consumed as a supplement or food. But some researchTrusted Source suggests that it’s only small amounts."
"For Insomnia: In a small 2018 study, participants who took 300 milligrams (mg)Trusted Source of GABA an hour before going to bed fell asleep faster than those taking a placebo. They also reported improved sleep quality four weeks after starting treatment. Like many other studies looking at the effects of GABA supplements in humans, this study was very small, with only 40 participants."