I ALWAYS call ahead to restaurants and tell them "My child has a severe allergy to gluten--can you tell me exactly what is used to make your sushi and if it is gluten free?" I always make sure to find out what type of vinegar they use.
1) MALT VINEGAR
If they use malt vinegar instead of rice vinegar, the sushi will contain gluten. Malt vinegar is made from barly and it is not distilled so you need to avoid it if you are on a gluten free diet.
Some sushi restaurants sprinkle this powder on the sushi rice which makes the rice plump and glossy. Because it is impossible to know the source of this chemical rice enhancer I would skip any sushi that has added powder--although it may or may not be gluten free. Haru in NYC uses miola powder--although I have a friend with celiac who recommended Haru I would not recommend it.
Skip the crab because it can contain gluten. We never order crab just to avoid confusion.
4) SOY SAUCE
By far, the main thing you have to be concerned about is the type of vinegar used in the sushi. I thought I would do a post because the miola powder is something I just found out about.
The following are restaurants where my kids eat sushi (and I have confirmed they use rice vinegar and not malt vinegar.)
Lilly and Loo's NYC
Yuka (all you can eat sushi!)
Dean and Deluca Madison Market Location @85th Street/Madison Avenue