I mean they've just finished Red Dead, i'm sure they'll be working on DLCs for that then looking onto either making a Bully 2 or GTA 6. But.. seeing as GTA 5 is being updated regularly, I don't think they'd find it necessary to start a new gta 6 yet. Maybe a few more years won't hurt.
I assume we'll see GTA 6 around 2019 at the earliest, but more likely around 2020/2021. Like @Owen said, Red Dead has just completed development and is due to ship later this year. They're still producing content for GTA Online, and will be producing similar content for Red Dead 2.
It's highly likely that GTA 6 is already in development due to the fact it takes years to develop such a game, but it's unlikely we'll see it on the shelves in the next year.