Right now... I think I'm at I'll get eventually but not at launch. But that could change.
Reasons including
1. Will be expensive at launch.
2. While I liked GTA V and GTA IV, I'm just not as excited about the series as I once was.
3. I've got a sizeable backlog games I want to get through.
4. I just lack the time to play games. In general it's one or two games per year, dotted with Mario Kart and Smash Bros.
Off the top of my head my game back log is around 20+.
If I somehow cross 6 games from that list before GTA VI launches and I'm on top of my life in general I'll buy it at launch. If not, I'll purchase later.