While most gambling sites accept Bitcoin and a couple of established altcoins too (Litecoin, Dogecoin, Ethereum), in some cases Bitcoin seems to get more attention when there are promotions.
(I did gamble with Dogecoin, but lately I use only Bitcoin for that.)

Then there are the virtually unknown virtually worthless altcoins used in certain games, built exactly around those coins. But that's in gaming sites not in gambling sites.