I’ve tried some of the GF beers and not found them flat. My guess here is that since most people don’t need to worry about the gluten, those GF beers sit on the shelf longer than regular beer, and since there’s not a space-shuttle quality seal on any of the bottles, they go flat with time. Instead of a particular brand, you may wish to check with the store’s beer buyer to see how long those have been there, although that won’t necessarily account for all the time it spent in warehouses somewhere. The fresher the beer, the less likely you are to be disappointed with it.

Where do you buy them? Perhaps a “specialty” store (like Whole Foods) where gluten-sensitive people shop, will have higher turnover (thus fresher).

I actually haven’t had any gluten free beers, but if you search around on ratebeer.com you’ll have some suggestions.