We have done GFCF (and also soy and corn free) since my DS (3) started solids. I noticed he had some terrible colicky reactions to all of these in my breastmilk and went off of them completely. Amazingly enough, it started me on the road to healing as well.
The few times we have accidentally or purposefully allowed my son to eat gluten or casein, he has been a mess. He already has sensory issues and they are uncontrollable when he is not on the diet. Crying for half an hour over something small, rage, hyperactiveness - totally uncontrollable.
I heard somewhere (I think it was a book I read about autism) that 80 percent of autistic children have some improvement on the GFCF diet.
I also heard it takes quite a while to get the gluten and casein out of the system, so don't lose heart if it doesn't work right away. I think for casein it was 2 weeks and gluten was even longer than that.