I normally play RimWorld in window mode with G-SYNC enabled. Not matter what setting G-SYNC is on the game screen flickers slightly. It seems like it gets brighter and darker every 1-2 seconds. When I disable G-SYNC the problem goes away complete. This only happens in RimWorld.
It looks like there are some issues with g-sync in Unity. I'm not sure there's much we can do other than hoping that updating Unity will fix it. Thanks for reporting though.