Rockstar has not yet announced GTA 6, but, according to the former artist of the studio, is already working on the game.
What is known
Michael Bibin updated the page on ArtStation,adding a new job and another project to your list of achievements. Now Michael works at Codemasters, drawing cars for GRID and F1, but until April 2018 he worked at Rockstar India, where he created auto models for GTA 5 and GTA 6. Michael's name is also on the Red Dead Redemption 2 developers list.
At the same time, the artist's profile on LinkedIn is deprivedinformation about the projects in which he participated. Rockstar monitors for leaks and does not allow employees to talk about projects, so the mention of GTA 6 can be considered a violation of the contract. Also note that in January 2019, the Indian division was looking for an “automobile artist”, perhaps Michael caused the lack of personnel.
Rockstar is famous for its slowness ingame development, so the GTA 6 release is not worth waiting for. The great auto theft games are usually done by Rockstar North, and work on the Western RDR 2 is given to the San Diego unit. Preparatory processes are taken by smaller studios, and it seems that the office in India has already begun to create some details of the environment.