Gluten is harder, in terms of avoiding cross-contamination and unexpected places (or maybe that's just what I messed up more, so I'm projecting that it's harder
). Some deli meats have traces of dairy (separate from the ones cut in the deli area that could be cut on the same cutter as blocks of cheese). It's xxx lactate IIRC, which is sometimes _not_ from dairy, but either they need to specify, or you need to call.
Um, odd processed foods that you may not use, I was surprised to see dairy in nonstick spray, I'm sure there are other things. The chocolate chips at WF (their brand, I mean) are all x-con with diary despite the warning that says "Good manufacturing practices used to segregate..." blah blah blah. Very disappointed in them. Enjoy Life are top 8 free, not sure about other brands.
And if you google GFCF diet and ASD, you'll get tons of hits.
eta: given how problematic dairy is, if that's what you suspect, I don't think it's worth testing, just do it and give it a while to see what symptoms clear up and what remain, and then you can decide whether to trial some dairy products (goat's milk kefir seems great for some folks, I haven't tried it yet for us but I'm considering it fairly soon for my daughter) and see what happens.