I tried some similar gaskets a few years ago, not that brand, but they were absolute crap.
For a start they didn't hold up in the carb, so fitting was a pain, and when I finally managed to fit them they just compressed right up to almost nothing in a couple of days.
Both Nitrile and Viton should be OK in petrol or ethanol , it is the other stuff they put in, including toluene, which causes the problems.