In NV control pannel, go to change resolution and use the 3840 x 2160 option under PC instead of under Ultra HD, HD, SD. It gives the option for 120 hz there.
Hope that helps
Im still getting screen tearing with the setting on. I'm getting mixed messages on whether one can get gsync running at 4k 120 hz on the c9.
I have a 3080 ti, ryzen 5600x and a good 8k hdmi 2.1 cable
Thanks