Where your FPS is lower than the maximum refresh rate of your monitor, G-Sync lowers the monitor’s refresh rate to match your current FPS. Where your FPS is higher than the maximum, G-Sync has no work to do, and you will get tearing. To avoid this, you will need to use something in addition to G-Sync - either traditional V-Sync, nVidia‘s Fast Sync, or a framerate limiter (such as Rivatuner).