The Diamond Casino isn't an area players can instantly travel to, unfortunately, as they'll first need to acquire a membership. Players can buy a basic membership to the Diamond Casino forR$500 in-game. This gives access to the basic gambling games and the ability to store one care in the casino's garage.
Cause you enter the casino through the front door next to the Valet or by using the elevator in the parking garage. If you're playing GTA Online and it's not letting you enter the casino, try restarting your game cause it may be a bug. The valet at the door initially lets you in...