I think others have covered the economies of scale and niche products creating the disparity.
But I wanted to suggest that if your grandpa is regularly eating gluten free bread, we have found that making it at home is SO much more affordable than buying a loaf at the store. (Even though gluten free flour is also more expensive.) Most of the gluten free flours have their own sandwich bread recipe, either on the bag or their website. I don’t know what flours you have access to, but they can be wildly different blends, so using their tested recipes is always best.
We’ve mastered our favorite so it takes only 15 minutes of “work” and then just time in the oven. It’s also much better than store bought! I don’t know if that’s possible for you, but it could be a lovely weekly ritual for you and your grandpa.
Also, to anyone suggesting we just eat rice and beans, I’m an old celiac. We went without bread, pasta, cake, pastries, cookies, brownies, pizza, and crackers before these products came to market. These are mostly “fun” foods that I don’t eat regularly, but usually pop up in social situations. Do you know how many sad birthdays with no cake we’ve had? How often we’ve watched our friends and family eat things we could never have? I am so grateful to the “fad” gluten free people who made it possible to have culturally/socially important foods we were missing out on for decades!!