Here's some info from the Mayo Clinic:
Allowed fresh foods
Many naturally gluten-free foods can be a part of a healthy diet:
Fruits and vegetables
Beans, seeds, legumes and nuts in their natural, unprocessed forms
Eggs
Lean, nonprocessed meats, fish and poultry
Most low-fat dairy products
Grains, starches or flours that can be part of a gluten-free diet include:
Amaranth
Arrowroot
Buckwheat
Corn — cornmeal, grits and polenta labeled gluten-free
Flax
Gluten-free flours — rice, soy, corn, potato and bean flours
Hominy (corn)
Millet
Quinoa
Rice, including wild rice
Sorghum
Soy
Tapioca (cassava root)
Teff
It doesn't seem like it would be hard to do this on a cruise ship or ashore. But I'm not celiac.