https://www.gamespot.com/articles/that-grand-theft-auto-6-release-date-in-gta-5-is-f/1100-6460150/

Grand Theft Auto Online players have reported receiving in-game messages stating that Grand Theft Auto 6 will launch in 2019. Developer Rockstar Games has addressed the rumors, confirming the messages are fake and not from official channels.

GTA Online players reported receiving the message, which reads "Rockstar Message GTA VI Coming 2019," on Reddit and the GTA Online forum. This inevitably prompted people to ask the Rockstar Support Twitter account about it. The response follows: