If you have a G-Sync compatible graphics card and a G-sync compatible monitor then you shouldn’t need vsync on at all and the monitor will refresh when its told to by the graphics card.
I had big problems with flickering too though when trying to use G-sync but I found that the problem was g-sync not enabled in my monitor settings.
I have a Samsung Odyssey G9 and there are two settings that sound sound similar - 1) Adaptive Sync which was on by default and 2) VRR control (variable refresh rate control) which was off by default. Once I turned VRR control on I had no tearing or flickering anymore and Gsync seems to work great.