Although nothing is officially announced, it is certain that Rockstar already working on a new GTA (GTA 6 ?). There is no need to fake surprise with this statement. However, here is a CV confirms the development of GTA 6 in a more or less convincing way.
A 3D artist named Bibin Michael recently updated his resume and list among his experiences, vehicle design for GTA 5 DLCs, but also for GTA 6. And bim, this is like shaking a surprise that will not be one during your ad. Interestingly, the guy worked potentially on GTA 6 between December 2017 and April 2018. So in the worst case, it's likely that GTA 6 has been in development since at least April 2018. But since a GTA requires many years of gestation, it is likely that we will not hear a lecture anyway for several (long) years …
To add a bit of credibility to all this (and prove it's not a nasty troll), know that Bibin Michael has actually worked on Rockstar since it was mentioned on the Red Dead Redemption 2 thanks page created by Rockstar on the official game site.