It depends on the games you play and how heavy Gfx you want to see and how fast paced the action AND how important your FPS are. If you are willing to dial your games down, do not mind giving up one or more of the above VSync or running without it is perfectly fine. G-Sync is for gamers (with extra coin) to see what their Rig can do. I, myself did alot of research on this and others gave really good answers in other forum. Do you want a Ultra Wide Curved or are you running 2 or 3 monitors?
To break this down another way: 1080p is different than 2.5k(2560 x 1440) which is different than 4k (3840 x 2160) Unless your sporting some serious heat, Dual 1080s or a Titan or Newer 1080 Ti, 4k gaming with High settings is not going to happen in any kind of real way. For myself I went 'middle of the road' with 2560 x 1080p with an Ultra Wide Curved w/G-Sync up to 200hz refresh. I wont go into what i bought but I'm enjoying the money I spent inside and outside of my Rig. Immersion is there and with G-Sync I'm able to play some fast-paced FPSs without screen tear, blur or loss of FPS. Right now there aren't alot of monitors out there that really do this well and the few that do you're paying alot.
Do the research and match up your monitor with what your Rig is capable of doing. To overbuy thinking you'll get what you want riding on the outside edge of your Rig's performance is a mistake alot of people (including myself) have made in the past.