There's a club3d Displayport to HDMI active adapter that should let you enable 4k120hz on the C9 with a 1080ti but it seems to not be too reliable unfortunately, reading comments about it: for some people it works, for some it doesn't, for some people works at first and then it doesn't.
But this adapter won't let you use G-sync on the C9 either, since afaik G-sync over DP works differently than G-Sync/VRR over HDMI
Using that, even if successful, would also mean that there's no HDR, correct?