We have cruised 7 times on NCL. My daughter has celiac, and she has never once had an issue. Fill out the forms. Book all of your dinners in the same place, same time. You’ll get the same waiter and table each night that way. At the end of dinner, they will give you the menu for the next day. Easy peasy.
The first meal is a bit difficult, but ask for the head chef at the buffet and they can walk you through every thing (or go to OSheehans but it’s a lot sparser here for GF). Good luck!