Vitamin D refers to either cholecalciferol (D3) or ergocalciferol (D2). I invite you to simply do a quick search.

If the GABA dysfunction is the chief problem, then it validates what anti-vaxxers have been saying:

GABA receptor-channel complex as a target site of mercury, copper, zinc, and lanthanides.

Mercury chloride modulation of the GABAA receptor-channel complex in rat dorsal root ganglion neurons.

In vitro and whole animal evidence that methylmercury disrupts GABAergic systems in discrete brain regions in captive mink.

Mercury interaction with the GABA(A) receptor modulates the benzodiazepine binding site in primary cultures of mouse cerebellar granule cells.

Functional deficiency implies that concentrations are normal, but neurological physiology is hindered in such a way that for all functional purposes it might as well be a deficiency. Taurine, glycine, theanine, biotin, pyridoxine, niacinamide, thiamine, magnesium (baths are really the only effective method for elevating blood Mg) are all supplemental GABA agonists. As I said, BCAAs help by depleting aromatic amino acids including tryptophan, but since we don't want to really deplete dopamine, supplementing tyrosine might be required. Some of these double-up in their therapeutic effect by acting as NMDA antagonists and blocking glutamate activity, especially magnesium. Without GABAergic activity, you get glutamatergic dominance i.e. hyperexcitation, which is a common term applied to autism neurophysiology. Ketamine might be a viable treatment but I don't think anyone has investigated this vein.

Some of the most powerful allosteric modulators of GABA receptors are the inhibitory neurosteroids: THDOC, allopregnanolone, 5a-DHP, progesterone, pregnenolone. These are contingent on healthy metabolism (high carb, low fat, high oxidative respiration, low glycolysis and beta oxidation), since they are regulated by steroidogenic enzymes. Simply supplementing them doesn't always help.

The gut-CNS axis is important too, since the epithelial cells lining the gut produce the majority of serotonin. I've read that some mothers achieve tremendous improvements on gluten restriction alone. Charcoal, bamboo shoots, raw carrot are good for adsorbing troublesome toxins. Cyproheptadine is promising:

Increased platelet 5-HT levels were found in 40% of the autistic population, suggesting that hyperserotonaemia may be a pathologic factor in infantile autism.

Serotonin was originally called enteramine since it was implicated in gut function and dysfunction. It was also called thrombotonin since it was implicated in circulatory disease, vasospasm, hypertension and promotion of clotting, until 1951 when they discovered these two things were the same substance.

Vitamin D is written as D3 for a reason, they're the same thing.

The overmedication doesn't worry me as much as the stuff people aren't aware they're doing. Small things that actually matter a lot. Like eating cereals containing iron, restricting salt because they think it lowers BP, looking at fluorescent screens and destroying their dopaminergic neurons. It's the decades-old advice the medical cabal still peddle and the laxity to do new work to maybe challenge existing paradigms. The level of complacency is staggering, within science as a whole. Did you notice everyone is afraid of the sun now? Through almost the entire 20th century, it was impossible to get anyone to believe that radio waves or visible red light had any biological effect on living cells. How rapidly a consensus can change...