Magnesium and minerals/trace minerals in general. Majority of them increase GABA and having calming properties specifically magnesium and they all act in balance/with each other. If you want to use magnesium in isolation magnesium threonate is the most effective for brain issues. Mag chloride works to it just takes long to build up in saturation since it goes all over the body.
Magnesium potentiation of the function of native and recombinant GABA(A) receptors - PubMed
The Role of Magnesium in Neurological Disorders
Treatment Of Magnesium-L-Threonate Elevates The Magnesium Level In The Cerebrospinal Fluid And Attenuates Motor Deficits And Dopamine Neuron Loss In A Mouse Model Of Parkinson’s disease
Magnesium L-threonate prevents and restores memory deficits associated with neuropathic pain by inhibition of TNF-α - PubMed