I got my g-sync monitor up and running. I still use my older monitor to view windows desktop.
Playing one of my games in full screen windowed mode, I noticed that when I was in game the fps dropped to 21/24. But if I moved my arrow to the 2nd monitor the fps in the game shot up to 71. Moving the arrow back to the game monitor and selecting the game reduced the fps back to 21/24.
After some messing around and searching I found a thread on the net that mentioned its a bug and to workaround it by going full screen in game and then back to fullscreen windowed, and this works. I'm now getting better fps all round. In-game im getting 100+ fps, and when on the other monitor I'm getting 87.
There are other work arounds too that I havent tried. Apparently turning fast boot off works, not sure why. Also someone said to download nvidia inspector and edit/create a profile for the game setting fullscreen and windowed flags.