It helps some (helps ds1), but I know others it has had no effect on.
You could try it - I would expect to see a fairly quick change if its going to make a difference (it was a change within days for ds1). You could also have the urine test at the SUnderland Autism Research Unit to see whether it's likely to help.
I've been buying it for 7 years so I don't really notice. I make my own bread, and tend to do a lot of cold meats and potatoes type teas now rather than specific gluten free stuff. Also ds1 has stopped eating so many biscuits (not completely) so he has things like apples or crisps instead, so again not specific gluten free.
This is all very recent- for years he would only eat a very limited diet and then things were pricey.
Sunderland test here: http://osiris.sunderland.ac.uk/autism/ (sorry, can't do links).
You could order gf foods online from Goodness Direct, who have a good stock and are fairly reasonable.
Be careful though, some supposedly gf products aren't totally gluten free - there's an acceptable permitted level for some items and I found some brands unusuable (Juvela for instance) because they weren't entirely gluten free. You could get a copy of the Coeliac Society's handbook, which lists all products available in supermarkets (including own brands) which are acceptable in a gf diet - last time I bought it I think it was £7 and is renewable annually with monthly updates.
As others have said, you're best off making your own bread, cakes etc which are always much nicer than shop bought.
I would suggest anyone who has a child with asd should try the diet. We were told by John Richer at the JR to try GF and caesin (dairy) free as our then 4 year old son had the tell tale signs of red ears, and pasty face and a craving for foods with gluten in. (He had gone autistic overnight at 15 months, refusing most foods suddenly, apparently this was when the level of opiates fronm the gluten and caesin tipped him over so to speak). He had been predicted to go to a special school, but when the Ed psy saw him a month later he suggested mainstream with an LSA! Gym, nursery, all noticed a remarkable difference. 4 years later we were able to have dairy and two years ago (age 13) he was able to come off the diet, (I have discussed the reasons for this in another thread)previous atttempts had meant immediate stomach pains and extreme fatigue. We were lucky enough to have a doctor who allowed us to get food on prescription, found the only decent GF/DF bread was the PKU bread by Lifestyle, they have a website.
Have you considered reducing Wheat? I find that I eat very little wheat. Eat quite a lot of Rice, Oats and Potatoes instead. If you don't want to go full Gluten free / find it hard to obtain/make Gluten free products, then I would suggest reducing Wheat and seeing what difference there is.
I'm self-dx AS, and find that I'm much less irritable if I eat little Wheat, less bloated, and seem to concentrate better.
Experiment a bit, see what works best for your DD. Give it a try for a month and see what differences (if any) you notice. Then try something else and see if that makes a difference.
Note: Phasing wheat out completely may not be possible - it can be used as part of flavourings. I would suggest just reducing the bulk of the wheat - so no bread, cakes, pasta. Also some drugs contain Wheat - Ritalin for example.