I decided to look up “ass gasket” to see how many places defined it as the paper toilet seat liner, and stumbled across an actual non-toilet article that they decided to use the name for:
It’s something you can strap onto your butt (so you don’t have to take off your ski boots or anything) on top of your underwear/long johns but under your outerwear/ski pants. It’s intended to keep your butt dry in the snow.
I’m not shilling for them, or anything. I’m just amazed. Clever marketing.