When did native Americans get permition to gamble?

Native Americans have Indian Reservations. These reservations are considered their own place and they don't have to follow all the rules of the united states, because technically Indian Reservations are their own places. They build casinos because casinos are illegal to have (unless in Las Vegas) because gamballing is technically illegal, but since its on Indian Reservations, it is allowed. They actually make a ton of money from creating these casinos on their Indian Reservations. Hope I helped answer your question.