my dd1 is also gf/cf
we put her on the diet when she was coming up to 3. she is 6 now, and has improved and changed so much.
I too have met nothing but scepticism form medical professionals, and even direct hostility to the diet (dd2's notes have "mother insists on keeping dd2 on a low fat diet" written in them, because I refused to give her cow's milk products when she was an infant
)
the AiA/Respetrum site is very informative, as is the Sunderland autism research Unit (now called something else, i believe, but you can find it by googling)
we did the Sunderland test with dd1, and she scored off the scale for gluten intolerance. not so much for dairy, but dh has a dairy intolerance anyway, so we pulled dairy for that reason. the differences have been huge for dd1.
she didn't have that much wrong with her bowels (or so it appeared) as a toddler - a little loose, stools-wise, but not too major. but this continued on as she got older, and it just wasn't right, iyswim?
anyway, that is all much improved now too.
agree with pagwatch - you have to stick to it scrupulously, and it is tricky in the beginning.
but one you get your head around the fact that, other than "real" food (ie non-processed) you won't be able to find stuff in the shops easily, then it gets better. dd1 can't have a lot of the replacement gf/cf foods because they are full of other stuff she can't have (sweetners/flavourings/MSG, even things like apples/apple juice - but this knowledge was built up over years)