It is not called GTA 6, which is why in the official announcement Rockstar referred to it as "the next Grand Theft Auto" rather than just saying "Grand Theft Auto 6".
The new game is called Grand Theft Auto™ and will be a full reboot of the series from the ground up. The intention is that Grand Theft Auto™ will be a final GTA game and it will last forever as a live-service game, simply being continually updated with new stories, missions, cities, and online content as the years go by.
This means that in future you will not for instance be buying "GTA 7, set in Liberty City", you will instead be buying something like "Grand Theft Auto™: The Liberty City Saga" as an update to the game you already bought.
As the leaks already revealed, Vice City will be the base city + storyline in the game at launch but more will be added in future and you will have to buy them all separately (for usage in both single-player and online play).