Any food in its unprocessed state (except wheat, barley, and rye) that is naturally gluten-free. So things like meat: chicken, beef, lamb, pork, etc. Fruits and vegetables: apples, oranges, pears, berries, peas, avocados, tomatoes, lettuce, artichokes, etc. Also, beans, nuts, and eggs. Most dairy products with one or two ingredients like milk, butter, cheese, etc. Read labels of anything with more ingredients. It is rare, but sometimes fillers and/or binders are used that may have questionable ingredients in things like yogurt, light sour cream, etc.
Now, here is the tricky part with even these items: fanciness. Seasonings, flavorings, dusting with flour to make things not stick together. In the grocery store, question seasonings, etc. In the meat department if something is marinaded or seasoned, ask the butcher and/or read the ingredients, if listed. Sometimes things like prunes are dusted with flour to keep them from sticking together.
*Anything processed and/or in a package needs to have the ingredient list read.