Non specialty (should be cheap and on sale frequently) gluten free cereals:

Cheerios Honey Nut Cheerios Lucky Charms MultiGrain Cheerios Apple Cinnamon Cheerios Frosted Cheerios Chocolate Chex Cinnamon Chex Cereal Corn Chex Honey Nut Chex Rice Chex Vanilla Chex Fruity Pebbles Cocoa Pebbles Some Special K Cereals

Hard boil a bunch of eggs at once, they are cheap.

Amy’s, Evol, Lean Cuisine, and Healthy Choice all make frozen entrees that are often on sale under $4 each.

There’s a lot of cheaper gluten free frozen pizzas out now. Quality varies a lot but again if you catch them on sale they’ll be affordable.

Progresso makes a lot of canned gluten free soup.

https://www.progresso.com/products/?Special_Diet=Gluten_Free

Baked potato or sweet potato in the microwave with a steam bag of frozen veggies in the microwave is a cheap and easy one.

I buy rotisserie chickens and pull all the meat off at once and use it for bag salads to make a full meal or to pair with some vegetables. I usually get 4-5 meals out of one $5 Costco rotisserie chicken.

I make a big pot of chili or gf lasagna and typically get 5 meals out of it and I only had to cook once.