I love love love Sushi and don't let my GF diet stop me from eating it. Luckily I love to eat the real sushi more than I love the specialty "vegas" rolls (because they are battered).
Happy Sumo (in Orem & SLC) was happy to make sure I got what I needed and even had soy sauce packets available (which is unusual). I just went right up to the sushi chefs, and asked them if they could prepare my sushi separate from the other rolls, and to make sure to change gloves. They were happy to :)
My only tip: Bring your own soy sauce! I've read that people always have some in their purse because they never know when they're going to be at a place where it's needed, but can't get GF kind because they either might not have it, or they won't know what you're talking about.