GABA (Gamma-Aminobutyric Acid) is a neurotransmitter (brain chemical). There is some evidence that its function is altered in PMDD. For instance, one study found that brain levels of GABA levels increased from the follicular phase to the luteal phase in women with PMDD, while GABA levels decreased from follicular to luteal in women who did not have PMDD (Epperson et al 2002).
Another study found that women with PMDD had lower brain GABA levels than women without PMDD (Liu et al 2015). These were both small studies, so more research is needed to find out if and how GABA plays a role in PMDD.