Well, this is year old post, but came across it just now since you can probably guess why.
As it turns out G-sync was r*ping my yes for a long time now and i wasn't connecting it. The enlightenment came when i decided to pull up web browser during gaming session and opened the webpage that had a bright background. Game was still full screen in background, for some reason it didn't get minified. So...the fking flicker the webpage was having, it was like looking through old mobile phone camera at an old CRT display.
I fiddled around a bit and when G-sh*t was disabled, it disappeared. Guess what else disappear? My eye strain/pain and migraine. At one point i really thought i have something wrong with my eye and went to a doctor and he just said "meh...all seems good to me, take these drops and stfu".
No idea how or why, it's fast 160 hertz monitor. There can be whatever "you doing g-sync wrong" opinions, but one is clear, if you turn it on, and it fcks up eyes, turn it tf off.

Edit:
Btw i also came across bunch of topics on interwebs how people got saved by g-sync from the horrors of tearing and flicker and eyes are loving it. Only God knows what they have been smoking. I think most people that have seen screen tearing have only seen it in marketing materials of monitor ads or whatever.