As far as Grand Theft Auto 6 news and rumors go, there is barely anything that we can say we know for sure and a lot of unsubstantiated hearsay floating about. Beyond knowing that it exists and will appear, sometime, probably, nothing is clear. However, a recent Instagram post may have shed some more light on the future of the franchise.

Hip-hop group City Morgue posted an interesting image to their Instagram story that teases an apparent collaboration between themselves and Rockstar Games. Known GTA leaker Yan2295 then signal-boosted the post in the community, kicking up a bit of a storm.
There isn't much to go on. The image appears to be one of a blood bag with the Rockstar Games logo, a large X, and the City Morgue logo on it. It is captioned "Summer 2020" and has the official Rockstar Games account tagged in it. The post refers to Righteous P, a member of City Morgue who initially posted the image.

When a fan contacted Peter "Righteous P" Rodgers, the response didn't reveal anything more, just that there would be an announcement "soon". The most likely assumption is that the group is involved with the soundtrack of an upcoming GTA game.

There are, of course, other explanations. Rockstar themselves haven't said anything, so there is a small possibility this is all a publicity stunt without any truth to it. It is also possible they are collaborating for other purposes, or for another game.

A rumor about a possible Bully sequel has been floating around the web for a while now, so there is a chance City Morgue is involved with that instead of GTA 6. But their musical style best fits the flagship crime game rather than the satirical school setting of Bully.