Assuming you gamble on a mid to high-rated gambling website and you won big but they refuse to pay up, what and what are the legal way to make sure that the platform pays up your money?
If it is a licensed casino, then you maybe able to take legal steps. Contact their license provide and file a complaint. If the amount is significant and you don't receive any positive response from the license provider or the casino, then contact your local law enforcement team, they will be able to help you with something or provide you with some guidance. If it is a non licensed casino, you likely won't be able to do anything. But in both cases, make sure to open disputes on various gambling related forum. If the casino cares about their reputation, they will likely return your winnings and funds (if you won without breaking any ToS).