Looks like it may modulate the release of multiple neurotransmitters, so you may want to hold your horses on the placebo study.
https://www.frontiersin.org/articles/10.3389/fphar.2020.0127...
https://www.frontiersin.org/articles/10.3389/fphar.2020.0127...
If you are low in GABA this will make you feel less depressed. If you are high in GABA it will make you feel more depressed.