I have the same issue, turn off and back on the G-Sync in the Nvidia control panel and go to a game - it should get fixed. To make sure G-Sync is really on (no matter what the Nvidia indicator says), enable the refresh rate count on your monitor. If the refresh rate matches your FPS, it means G-Sync is enabled. If it says constantly at your monitor's refresh rate, G-Sync isn't working.
I don't know why, but for me, if you close Nvidia control panel, you will have to turn G-Sync off and on again.