I've been disappointed with Nizza in recent visits. You should try S'Mac at 157 East 33rd Street between Lexington and 3rd avenues. It is gluten-free mac and cheese with dairy-free options too. After S'Mac, you can get dessert at Pip's Gluten-Free Bakery on 39th Street. If you are looking for more sandwich-type meals, try Bloom's Deli or Friedman's Lunch.
Have a great time. NYC is a very gluten-free friendly city.