I there are already cryptocurrencies that are been using in the gambling sites
Yes, like the projects that are coming out now. They are making their casino with the token that they are also using to fund their business.
That kind of strategy is not proven successful, most of the successful casinos now are just using the traditional style where they accept major coins and altcoins. Maybe gamblers are not trusting too much on gambling sites that use their altcoins because anytime the value could drop since their liquidity is not that good, unlike with BTC and ETH where you can trade them at any trading sites in the space.
This is the list of gambling sites that uses ICO, I'm familiar with some of them but I know they are smaller compared to the popular gambling sites here accepting bitcoin and altcoins.
https://www.katiewager.com/gambling-tokens.html