The full answer
Wheat, barley, and rye are the main sources
Gluten is found in wheat (including spelt, durum, semolina, farro, and einkorn), barley, and rye. Because these grains are used so widely, gluten shows up in bread, pasta, most beers, many cereals, and countless processed foods as a thickener or binder.
What's naturally gluten-free
Rice, corn, quinoa, and potatoes are naturally gluten-free — see can people with celiac disease eat rice and can people with celiac disease eat quinoa. Oats are a special case worth reading about separately, since cross-contact during processing is common even though oats themselves don’t contain gluten.
Important context
Individual symptoms, reactions, medical histories, food products, and local labeling rules can vary. Community experiences are not medical evidence.
For personal medical decisions, diagnosis, testing, or treatment, consult a qualified healthcare professional.