I just got a 240hz gsync monitor and having it on vs off is night and day to me.
I know this because it bugged out on me when I was playing overwatch and it was disabled. I believe it had something to do with me switching windows and having OW in windowed mode. But there was defiantly noticeable tearing (getting around 180-220 fps). Getting it working again made it buttery smooth again
Like others have said, the benefit comes from the frame rate dropping below the refresh rate of the monitor. If you are pushing enough frames to your monitor to keep this from happening, then its literally not doing anything. In my case, having a 240hz monitor, I get the benefit of as high an fps as my computer and push (up to 240fps) without any of the issues you have when it drops a little bit.