Update #3: Leaker proves they have the game in some form. Multiple videos of what appear to be legitimate GTA 6 gameplay are currently being posted to the Internet by a leaker. I've seen two clips so far: the first depicts Jason at his house from the trailer playing some basketball, and the second features some driving and crimes that attract the attention of the cops.

I will not be linking to either video, but they are easily found through social media searches at the time of writing.