I was getting full screen flickering (even though Plex was in a window) and glitching and what seems to have worked for me is to create a custom profile for Plex in the Nvidia control panel. Go to 3D settings, Program Settings, Add and choose Plex (have it running already and it will show up in the list). Now scroll down to Monitor Technology and change it from Gsync to Fixed Refresh. Hit Apply. This will keep Gsync on for everything else except Plex.