This building was made accessible with the 12/22/16 update. You can enter it through the doors top north of the Oasis Area.
There, you will find various casino games (though for now, only slot machines are playable). The player will bet money and win or lose randomly.
It is not a very efficient way to make money, and you can actually lose some if you're unlucky, whereas the other minigames will only help you win some.