On thing I have found with GSync so far is that for the game Hearts of Iron 3 the frame rate is absolutely tanked whenever the the hard disk is thrashed, e.g. loading a save. The screen will flicker and the frame rate drops to single figures. Don't know how widespread this is across games with similar demands, but anyway to the point... I have to disable GSync when playing this game. Also happened in EUIII, though not as bad.
Since I can't find an desktop icon or other easy method to toggle GSync, you have to do it in the driver control panel. You either have to disable it altogether each time you run the game, or set up a 3D profile such that when that program is loading it is switched off, since gsync is an option amongst all the other 3D settings. Makes sense to use the profile right? This works as expected, as long as that is the only program running.
However, if you have something on the other monitor which the drivers detect as needing 3D, such as Chrome, it gets confused. This means I can't have a Youtube session open on the other window whilst playing the game. It appears to override the profile and turn GSync back on and you get the frame rate problem in the game, and on the youtube video strangely.
I imagine this would also happen you you had two GSync monitors as well, but I thought this might be relevant as the idea of playing a slow strategy game whilst watching some youtube on the other monitor would be fairly common. Don't know if this is restricted to older games such as these. I would be interested to hear if someone has solved this, or has found a way of toggling GSync with a desktop icon or something, to make it quicker.