I've noticed that alot of people are using g-sync incorrectly (aka default settings), which will result in drastically increased input lag - correctly configured, g-sync has the equivelant input lag of v-sync off (which is the fastest method available).
What you need to do to configure it correctly :
1. Download Msi Afterburner, which includes Rivatuner.
2. Enable g-sync for fullscreen in nvidia control panel.
3. Force v-sync on in global settings in nvidia control panel.
4. Use rivatuner to limit your fps 3 below the hz of your monitor. So if you got a 144 hz monitor, limit the fps to 141.
5. Disable v-sync in the game settings.
Mouse input lag should now be 2 to 3 times less compared to g-sync at stock settings.
An alternative to using rivatuner to limit the fps, is making a user.cfg file with notepad, where you use the command GameTime.MaxVariableFps to limit the fps. This will give you slightly less input lag than using rivatuner, but at the cost of much more uneven frametimes, making the game feel more stuttery, thus i personally find it preferable to use rivatuner.