After years of speculation, Rockstar confirmed on February 4, 2022 that Grand Theft Auto VI was in development. The game carried the internal name Project Americas. Full production had begun in 2020, after groundwork since the end of Red Dead Redemption 2.
Wikipedia's development history fills in the timeline. Preliminary work began in 2014 after GTA V, and Rockstar's research team made its first trip to Miami in 2015. Early development started in late 2018 after Red Dead Redemption 2.
It is the first main Grand Theft Auto story written without Dan Houser, who left Rockstar in 2020. Rupert Humphries and Michael Wiafe are the writers, Rob Nelson is a producer, and the game runs on Rockstar's RAGE engine.