I was up in Mammoth two weeks ago and found a few gluten free places to eat. If you are up there for camping/fishing/biking/skiing/snowboarding/... here are some places to consider.
Stellar Brew and Deli
3280 B Main St.
Serve Breakfast and Lunch (coffee shop with alcohol).
They had many gluten free and vegan options on the weekend, and a few on weekdays.
I tried two of the muffins and enjoyed them both.
They also have GF sandwiches for lunch.
Great GF Option. Cozy eat in area, or take your food to go.
On the corner of Main St and Sierra Blvd, across from Quality Inn.
Wood smoked BBQ with several gluten free options. Gluten free options are not online, but ask at the reservation desk in the restaurant.. Has a slightly different taste since it is wood smoked. Their homemade bbq sauces were gluten free. Not overly pricey (~$20ish). I'd eat here again (and would probably enjoy the meal more if I were fresh off the slopes). Nowhere near as good as Blue Smoke in New York, or Newport Rib Company (both reviewed elsewhere on this site), but celiacs can't be all that picky when it comes to finding places that you can eat out in smaller towns.