While any talk of a casino in GTA Online has pretty much died away this past year, there was a time when it was one of the most pressing matters as far as the community was concerned.
Everyone was wondering when the "casino DLC" would be released, but it never game around.
And based on what's going on in the industry these days, that isn't a bad thing.

Grand Theft Auto 5 didn't make much use of the casino as a location, it only appears briefly during one of Trevor's character switch scenes when he's being tossed out for cheating. This seems to contradict the "opening soon" sign on the casino, which otherwise looks empty at all times.
When GTA Online launched, the casino didn't get any more functionality above the store mode, and that "opening soon" sign stayed right there. Players assumed this might be a hint towards some kind of DLC that adds a playable interior with actual gambling, or that it would be the location of one of the Heists.
Neither of these came to pass.
However the idea of a working casino with real gambling seemingly endeared itself to the community and players just wouldn't shut up about it for years. Even though the microtransaction angle allowing players to buy currency would make it close to real gambling, many thought a DLC would come.