Try to turn off VSync in MSFS and set VSync to “Fast” in nVidia control panel.
The problem is that if MSFS falls under the minimum VRR for your monitor (30 or 48 Hz) GSync turns off and you are left with the VSync setting of your card. As for the flickering, probably your unit is GSync certified but not for the “ultimate” experience. Check here: GeForce G-SYNC Monitors: Manufacturers & Specs