There has been a leak of a lot of footage of a development build of GTA 6 on GTAForums:
https://gtaforums.com/topic/98…eak-90-mp4-footagevideos/ Edit: The thread on GTAForums has been deleted.
You can find those videos on YouTube. Edit: however many of them are being taken down by Take Two.
It looks very legit at this point, it looks like the RAGE engine and even include voice lines, which makes a fake very unlikely in my opinion. Many assets are clearly taken from V, though these are possibly placeholders.
This also confirms the previously leak about being based in Florida (Vice City) and having a male and female protagonist.