G-Sync is good regardless of your frame rate. In fact, it's better when your frame rate is low than when it's high. If you can't maintain 60 FPS on a 60 Hz monitor, your options are either to turn off vertical sync and live with screen tearing, or leave vertical sync on and stutter back and forth between 30 and 60 FPS several times a second. With G-Sync, the monitor refreshes when a new frame is available instead of refreshing at 60 Hz whether or not a new frame is available. That means if you're averaging 56 FPS (for example), you're seeing 56 FPS.

The down side is that G-Sync is rather expensive to implement, so G-Sync monitors cost more. You're also locked into Nvidia GPUs, but that's less of a practical issue since Nvidia's not going anywhere any time soon.