I have not seen any gambling site till now who credited before being confirmed. And I think if any gambling site credited before being confirmed then the gambling site would not allow withdrawal untill the confirmation as well as the wager requirement. So, there is no way to be scammed by double spending.
Now this is the case for every website because otherwise every casino and exchange would be scammed until their hot wallets go empty. But in the early days with bitcoin starting and having less value and evil people eyeing, this would be the case for some websites like Satoshi dice.
For exchanges some like Binance accept 1 confirmation to trade but only allow withdraws after many confirmations to be even more safer.