1080TI can only do GSync over DP, I believe you need a Turing card or newer (for Nvidia at least) to get it over HDMI like for the OLED VRR.
Ta, knew were some caveats around forum VRR/ G-Sync compatibility etc hence initial question. Knew way back when I had my 1080Ti with the C9 it did not play ball, but not sure if it changed with newer models CX / C1, but as you point out, is more a limit on the GPU side of things in this case.