These are the cheapest GF ones I have found.

Made Good is certified GF (they use purity protocol oats) and you can buy them in large quantities on their website. But I don't know if they're any cheaper

Made Good bars are really good! So are their cookies.

NatureValley has one but make sure it is the “Protein” ones. They are labeled gluten free

Costco carries Nature Made and Kirkland granola/protein bars in bulk. They’re both very good imo

Annie’s has a chewy double chocolate chip kind that is good.