The footage was stolen from Rockstar UK offices and consists of various clips of unfinished gameplay and assets. The company has accepted that it is original in-game footage, but since the game is still in very early development, there is hardly any objective way to judge how the final product will look just from watching the leaks.
The person responsible for the leaks initially posted them to GTAForums, after which the news rapidly spread through the internet. The game developers soon filed copyright claims to get the footage taken down, but there are still undocumented copies of it circulating the internet. They are reportedly working with the authorities to ensure something like this doesn't happen again. In fact, there are claims that the UK police have already arrested someone in relation to this case. However, we have little information about the state of the legal proceedings since it is still an ongoing investigation.
What we do know is that various fans of the franchise have already started scouring the web for that footage in hopes of getting usable intel about the game. In fact, some fans are actively working together to create a proper map for the new in-game world, based on various screenshots and cutouts taken from the leaked footage. From what they have pieced together so far, the new map might be even bigger than the one available in GTA 5 and feature multiple regions with noticeable borders and geographical differentiations.
Chances are that the developers might make some aesthetic changes to the map to differentiate it from the leaks, but the overall open world will still be largely the same. So, the efforts of these dedicated fans might not go to waste entirely. The mapping project is still ongoing, and they will release the final render to other fans on the same forum, only in topographical format, so as to avoid posting any actual in-game screenshots that might get them in trouble. Let us wait and see how close they come to the original game whenever it ends up being released to the public.
