I've seen repeated studies--large-scale and well-designed--that say GF/CF has no benefit for autistic children. It has, however, been proven to cause bone-density loss.

I've seen repeated testimonials that it does have a benefit. Most likely, I think, people who say that GF/CF has a benefit are mistaking natural--often sudden--growth and development of autistic children as being caused by the diet; or else seeing improvement that happens in a lactose-intolerant child when milk is taken out of the diet (or a child with a wheat allergy; or a child whose diet improves in quality due to the GF/CF restrictions removing overly processed, low-nutrition foods).