Grand Theft Auto 6 is Under Development.

Rockstar has yet to shed any light on their next project Grand theft Auto 6.And in projects like this the developers works under NDA (Non Disclosure Agreement) so they cannot afford to share any information of the project but  accidently an Ex-employee of Rockstar leaked that that Grand theft Auto 6 is under development.




A former employee of Rockstar mentioned on his resume that "He has worked on creation of Concept vehicles matching with real world for Grand theft Auto 5 dlcs and Upcoming Grand theft Auto 6"




This artist (Bibin Michael) also contributed to RDR2 in some capacity, as his name appears in the Thank You page on the game’s official website, so he is indeed a former Rockstar Games employee, and the information on his resume is likely legit.
Bibin Michael has worked between 2017-2018 which means that Grand theft auto 6 has been under development for a year now and maybe we can expect GTA 6 before 2021. 
We haven't heard from Rockstar on this issue yet I am sure that sooner or later Rockstar will have to comment on this issue.