Rockstar has been very busy this year. Grand Theft Auto V has been one of the best selling games of 2017, Rockstar continues support GTA 5 Online, and Red Dead Redemption 2 is slated to come out this fall.
Additionally, a leaked online resume allegedly from Tim Neff, a motion capture actor who worked on GTA 5, suggest Rockstar may be even busier. The resume listed work on both Red Dead Redemption 2 and GTA 6 which is yet to be announced. Motion capture work would have been especially significant because it's generally one of the last things studios do when making a game which means the release could be relatively soon.
However, Tim Neff denied any connection to the resume shortly after it was leaked. He claimed it wasn't his resume and he was not working on GTA 6 saying on twitter that, "It seems I'm being trolled online really badly."
This is more in-line with the original understanding that Read Dead Redemption 2 would be out before GTA 6. Rockstar is most likely focusing on their current projects, and unfortunately for fans of the franchise, more reliable sources suggest that GTA 6 may not be out until 2020.