There used to be a site which compared the odds in sporting bets from a lot of booking sites and provided some bets that where you could bet on both teams (on different sites) and always have a profit. It was really small percentage but I consider this to be a loophole. I don't remember the site though.
Still this means there is only a loophole in the implementation of the gambling, not the gambling itself.
None of the well established gambling games have loopholes on the protocol level of the game. If a website does a stupid implementation or makes a mistake that allows people to interfere with the gambling protocol, than you it is not a gambling loophole, but a casino security flaw.
Best is not to look for exploits and just enjoy the games.