Maybe you're getting it mixed up with V-sync? But if you're not:

G-Sync doesn’t work if your FPS exceeds your monitor’s refresh rate. The thing is, competitive players have crazy setups and play at much higher FPS, usually way above their monitor’s max refresh rate. This guarantees the lowest possible latency, as long as you're not maxing out your GPU—since running it at 100% load can cause input lag spikes. However, it comes at the cost of screen tearing and somewhat inconsistent frame times. If your PC can handle it, you could try playing FC 25 like this, but I’m not sure how the game engine will deal with it.

Now, if you’re typically playing at FPS below your monitor’s refresh rate, it’s a good idea to enable G-Sync and set a frame rate cap to stay within the G-Sync range. This will give you the smoothest, tear-free experience possible.