The next GTA was announced in February, when Rockstar said that development of the unnamed game (which we're calling GTA 6 for now) is "well underway." Rockstar parent Take-Two said exactly the same thing today—development is still "well underway"—and added that the game is going to be really good, too.
"With development of the next entry in the Grand Theft Auto series well underway, the Rockstar Games team is determined to once again set creative benchmarks for the series, our industry, and for all entertainment, just as the label has done with every one of their frontline releases," said Take-Two CEO Strauss Zelnick during today's financial update.
following the recent Bloomberg report which claimed to have a few details about GTA 6, namely that it will include a female protagonist and take place in a fictionalized Miami. Rockstar hasn't confirmed or denied the accuracy of that report.
Bloomberg also said that GTA 6's scope had to be "reeled in" at some point, but that it'll be updated with "new missions and cities on a regular basis," so after release it may move back toward its pre-reeling-in scale. Even at launch, though, the publication says that GTA 6 will include more interior locations than any previous GTA.
The Bloomberg report also describes a Rockstar that has become a better, if not flawless, place to work in recent times—you can find the full article here.

GTA 6 will set creative bars 'for all entertainment,' declares publisher
Take-Two also reiterated that Rockstar's next big open world crime spree is "well underway."