I wonder where is the main source rate exchange between currencies for gambling website? Coinmarketcap?
It really depends on their management. Usually, they depend on a big exchange to base their rates. However, the problem with on-site exchanges are that it usually is with mark up to it. They charge a higher price to exchange than if you just bought it yourself over at the exchange you use. So, I would not really bother use these on-site exchanges unless extremely necessary.
There's no way to avoid this if OP is on a casino that accepts bitcoin deposit but they turn his bitcoin in to fiat once it goes to his account. Like 5dimes for example they use the current price on bitstamp to convert every bitcoin deposit to usd.
Cubits is another payment system that is being used by online casinos (just like bitpay) i've used them before but the gambling site I go to doesn't convert my bitcoin to fiat.