About a year ago, rumors of a GTA Online DLC related to the Casino with the "coming soon" sign hanging outside began circulating before being shot down on account of images allegedly showing the sign saying "opening next week" to be a result of mods. However, the fact that a Casino fits into the profile of a GTA game so well kept the rumor alive to some extent to this day, with many players expecting the doors of the money-sink to open at some stage.
Sorry to crush any hopes of making in-game fortunes off of gambling, but it will never happen. GTA Online's Casino will never open.
At least, it will never introduce any actual gambling minigames. At most it will be an open location where some mission takes us, maybe a new heist even — but there will never be gambling in GTA Online, for two very, very simple reasons.
First of all, it would be unprofitable. While at first glance, tantalizing players with a chance at easy cash without buying Shark Cards, only to have them lose their money, forcing them to buy Shark Cards may seem like a good business model, it wouldn't work. The conspiracy theorists out there would accuse Rockstar of purposefully making winning impossible and considering how vulnerable to hacks GTA Online is, the system would be exploited.
Secondly, Online gambling is quite literally illegal. In some countries or states, the mere possibility of real money being involved due to Shark Cards would make this illegal activity, effectively forcing local authorities to ban the game. Even though players have no means of turning in-game currency into actual money, only the other way around, the mere fact that actual cash can in some tangential way be related to an in-game activity which simulates gambling can make this thing a literal crime.
While Rockstar and GTA may thrive on superficial controversy about the virtual depiction of violence and sex, they hardly want to be associated with any *actual* felonies! While it is odd to include the Casino building and not actually do anything with it, that isn't grounds enough to assume that Rockstar will include unprofitable and probably illegal features into their flagship title.
While GTA Online does include betting on races, due to the indecipherable mess that gambling laws are, that is somehow legal under all jurisdictions that also happen to be selling GTA Online.
