Found a fix for this myself years later. Posting in case it's useful to anyone else
(This is Budgie specific but it might work with other DEs if you replace "budgie-wm" with your window manager)
- Open nvidia-settings as sudo
- Go to application profiles and create a new profile with GLGSYNCAllowed set to false.
- Create a new rule assigned to that profile and under Process Name enter: budgie-wm
- Make sure everything is saved. There's a bunch of save buttons and one is right above the Quit button under Application Profiles.
- Now go to the "X Server Display Configuration" tab and hit "Save to X Configuration File" and save.
- Reboot.
You can test if GSYNC is working properly in-game by enabling the indicator under OpenGL settings