Your Gsync monitor should have an FPS counter that you can activate in you monitor menu, mostly somewhere under “OSD” settings. If the monotor refresh-rate is changing along with your ingame FPS, it works.
My experience is that it works, kind of, but not constant. Gsync “disconnects” and the monitor goes to the native refreshrate and then “connects” again and so on… its a bad experience for me, stutters and flickers in MSFS. Even if i am in the FPS-Range that Gsync and my monitor supports.