I just started to replay the game after getting my save wiped out. This will be a good read. There is the little animation for it on YouTube. The video is Gravity Rush 2 - episode Overture.
Never heard of a manga about it. This just serves to show that Sony left the franchise to die back in 2017. It wasn't show in the main stage on E3, it wasn't marketed properly because Sony themselves cared more about marketing Nier Automata and Nioh (I own both games, and both are excellent).
I've heard about a possibility of a third game, but it is hard to believe that Sony will give Toyama (same creator of Silent Hill and Siren) and his team another chance with this lovely franchise.