An anonymous user recently posted a message on 4Chan imageboard (image forum). The message is very short: "March 25th. GTA VI."
The message is now deleted. Luckily, there is a screenshot.
Of course, this is an unconfirmed rumor, but it is worth noting that precise announcement dates of new games appeared on the imageboard before, for instance, Red Dead Dead Redemption 2 announcement date. Also, why delete a message if it's fake?