In fact, you cannot treat gambling as passive income in any sense. If you do not gamble, you won't have any chance for income on the other if you bet, you have no chance of winning you can make loss as well. However if you are staking on any casino site it would be your pass income otherwise we cannot adjust it with passive income.
Investing in casino tokens I think can be considered a way to generate passive income. Betfury is the kind of platform where they share the profit to the token holders. This way you can make money without having to gamble money out of your pocket.
There are several of this casino, its not however big if you invest small amount so same rules apply. The bigger your investment the bigger the passive income is.