The first thing to get straight about not on gamstop casinos is that they aren't a loophole - they're simply outside the fence. Gamstop is a free self-exclusion register: sign up and every UK-licensed operator must freeze your account and turn away new ones. Offshore sites never joined that arrangement, so they'll take a registration from a UK player while running on a Curaçao, Anjouan or Malta licence....
