Not much time to write but I will say that I'm on this forum for the same reason as everyone else here and am feeding my family gluten free, whole foods, and my toddlers breast milk like many are. However, I have worked with kids on the spectrum and have yet to find peer reviewed research on the topic (anyone?) or any changes in the dozens of severely affected kids I've worked with who went GFCF (maybe they weren't truly clean though?). IMO, the best you can do is work with your child as much as possible using well researched methods-ABA has thousands of studies in peer reviewed journals showing effectiveness but it takes A LOT of work to implement 24-7. if you have the money or insurance coverage, find a BCBA who will train you and your family to implement strategies rather than work with your child and leave. Please message me privately if you want to chat
Just wanted to mention that the GAPS book explains why GFCF only addresses a small portion of the underlying problem, which is why some kids see some benefit and some really don't. The whole underlying problem is an issue of extremely compromised gut health, and the gut needs to be healed directly. GFCF addresses two major problematic proteins, but may not do much to actually heal the gut itself.
The reason that gluten and casein are commonly problematic for people on the spectrum is that, because they have impaired gut function, they cannot fully digest the proteins. They are partially digested and you are left with glutomeorphins and casomorphins. Normally the digestive process would continue, but in ASD patients, it can stop there. Because the gut is so damaged that it's permeable and allows large particles to pass through, the gluteomorphins and casomorphins are allowed into the blood stream where they are able to affect the brain...and they happen to be neurotoxic. So when you remove gluten and casein, you remove these two neurotoxins, but don't really do anything to resolve the underlying digestive issue. A lot of people who go on the GAPS diet for a long period of time and completely heal their gut, can go back to eating gluten and casein (and other foods they were previously sensitive to) because their bodies are now capable of fully digesting them.