Honestly, I think it's helping. In D's case, we were looking to combat his violent and aggressive side. About a week after he went GF, the attacks on people in school stopped (he never did it at home), he's handling transitions better, he's accepting criticism well too. He never did those things well before the diet change. I think that if things keep improving, we'll push to Casein free too, though that is even more difficult to do.
The hardest part has been finding ways to make sure it's all truly gluten free. I carry a list of "safe" foods and ingredients when we go shopping and buy based off that. You'd be surprised what can contain gluten. Anything from "natural flavors" to "caramel color." Of course, the best option is to make sure it SAYS gluten free on the packaging.
Our saving grace has been corn pastas, some gluten-free meatballs we found at Costco, Naked Nuggets (also at Costco), and GF pretzels and cereals. Devin is a picky eater, so finding subs for his favorites is a true Godsend!
Read the book Special Diets for Special Kids (and it's follow up--same title 2). It's specifically for GF/CF, but it's worth reading even if you're just going GF.