Grand Theft Auto has always been a franchise which cultivated a large "leak culture". Whether it's about alleged secret easter eggs in older games, info about upcoming GTA 5 DLC, or in this case, about where the series will go next. The latest leak is a pretty tall tale about supposed multiple eras and locations for Grand Theft Auto 6, including non-American cities.
Let's preface the following first, by saying that this is almost certainly fake and not the direction Rockstar will be taking the series. If some bits of it here and there do turn out to be true, that will be an act of chance. In any case, you shouldn't go expecting the next GTA game to look anything like this.
The leak comes from two sources who posted to Reddit. The two accounts line up for the most part with some small differences in details, and both claim to have learned all this from insider sources working either for Rockstar or for notable gaming news sites like PC Gamer and Kotaku — both of which covered the leaks skeptically. Kotaku, in particular, is known for accurate, major leaks, so if they didn't post this first in spite of being an alleged source, you know it's bogus.
Release Details

GTA 6, codenamed Project Americas, has allegedly been in development since 2012 and will only be released for the next generation of consoles, the PS5 and whatever Project Scarlett (Xbox) will be called upon release. The game won't be launching on PS4 and Xbox One and won't act as a generational bridge like GTA 5. It's an all hands on deck production involving all studios. There is no projected release window yet.
Of these details, we can see it being a Rockstar Worldwide production and skipping current gen being plausible, but those can easily be guessed without any "insider info". Names change during development, so there isn't really any way to confirm or debunk that.