Make a list of foods she eats put them in one of four columns, "gluten free", "dairy free" "gf/cf" and then "gluten and/or dairy". Instead of trying to swap out foods, try to just eat what is currently gf and/or cf. Then add in other naturally gf/cf foods. gf/cf substations often are less healthy (more sugar, less fiber) it is better to stick to non to low process foods anyway.
With my son I was incredibly skeptical, but tried it and noticed within a few days he is more even tempered, after over a year he is much more even tempered, less irritable. and as a very unexpected side effect his dandruff cleared up completely and nothing even three different shampoos daily had helped that ;)

I found that whole foods and going to mom and pop bakeries, help with weeding out caseine, and gluten foods. Though it can be expensive. The benefits for some ASD children is that aggressiveness goes down, hyperactivity goes down. Sugar in their brains may create more impulses, causing them not to pay attention. Also implementing fish oil, vitamin A help with cognition. Hope this helps, also stay away from fruits that contain a lot of sugar, what's good for kids on the spectrum are pomegranate esque fruits

We are trying the Reid diet from unblindmymind.org. Very paleo gfcf. Daughter seems slightly more calm and focused 3 weeks in

CF is harder. I started introducing my daughter to almond milk, then switched to coconut when I noticed that there are more/better dairy alternatives made from coconut than from almonds. It's tricky because you have to stay away from soy. You can get coconut milk (the milk substitute in the carton, not the can in the Asian Foods aisle) and cultured coconut (yogurt substitute) at most major grocery stores. SoDelicious makes all kinds of coconut-based ice cream substitutes. I can't seem to get my kid off of cheese or milk-based yogurt, but she doesn't drink milk anymore and that seems to have made a difference.

Start by replacing what she normally eats with gluten free foods.