It was a breach of Rockstar's network that caused the massive GTA 6 leak. This is evident from a comment from the company, which at the same time confirms what everyone believed - the leaked material is genuine, and thus the very first material from the next GTA title.
The culprit(s) came across "confidential information" including "early development material from the next Grand Theft Auto," Rockstar writes. There have been reports that the source code is also on the loose, but this is not commented on by the developer.
"We recently suffered a network intrusion in which an unauthorized third party illegally accessed and downloaded confidential information from our systems, including early development footage from the next Grand Theft Auto.
Rockstar is disappointed that things leaked, but promises that work will continue at the same pace as before. They also promise a status update shortly.
"We are extremely disappointed to have any details of our next game shared with you all in this way. Our work on the next Grand Theft Auto game will continue as planned and we remain as committed as ever to delivering an experience to you, our players, that truly exceeds your expectations. We will update everyone again soon and, of course, will properly introduce you to this next game when it is ready. We want to thank everyone for their ongoing support through this situation.
The leak contained upwards of 90 clips from apparently early versions of the game. The interesting thing is that the footage seems to confirm previous information that the game has (at least) two protagonists - one of which is female. It also seems to take place in Vice City-like environments, which has also been speculated about before.
Rockstar has confirmed that they are working on a new GTA game, but it doesn't have an official name yet - as you can see in the quoted text, it's called "the next Grand Theft Auto game". It is also unknown when it is supposed to be released.