I didn't think Gamescom had much of a chance in all honesty. Then they released a trailer for it and at 1:50 we see a shot from our GTA V trailer. I don't want to get my hopes up because they definitely may be just trying to advertise their event by putting one of the biggest IP's out there in their trailer. But who knows? We'll need to wait for official confirmation.
Please don't take this as anything more than a gigantic rumour.
Edit: For people who say that Gamescom has no chance because Rockstar weren't at E3. They've done Gamescom before. Photos from two years ago: