Donna,
Gluten free is pretty easy to handle onboard a ship. My FIL requires a gluten free diet and has never had a problem with finding anything to eat.
Use the same sense you use in everyday life. No breads, breaded foods or pasta. Stick with rice products, meats, vegetable and if unsure ask. Also remember that many sauces are flour based (bechamel) and can give you trouble.
Take care,
Mike