In my experience with G Sync the on/off function was handled by the Nvidia Control Center app thingy. You would select the monitor and in one of the panes it would show that option where as when you don't have a capable monitor that setting wouldn't be there. I have not yet run into a game asking me within its settings console for that feature.
There are quite a few reasons your monitor may be "flashing" including whether you are using Windowed or Full Screen, etc. and also whether you are playing the game in the native resolution on the desktop or not.