I heard the reason why the 900 series and below isn't supporting GSync compatibility is because they only support DisplayPort 1.2, but FreeSync requires DisplayPort 1.2a, which NVidia only adopted on the 1000+ series. I imagine the old messy FreeSync hack still works, though probably with an older driver only:
https://www.techspot.com/article/1687-freesync-on-nvidia-gpu-workaround/