June 2019 Update: A casino has been officially confirmed for GTA Online. Whether gambling will be involved is not yet known. Full details here.
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.
