Well, the best way would probably be to limit your fps on some 140 number for it to never go above the gsync range. Or use fast sync and live with stutters. I don't know what gsync should show when fast sync is enabled so can't help you here.
Well it seems that g-sync recognizes fast sync in the sense that the game engine can render 200+ fps, but the monitor refreshes below the 144 cap. So somehow there is no input lag and there is also no tearing, but game logic renders at high fps. At least for me.
Some games stutter, some don't.
CS:GO: renders 240fps. Monitor refreshes at 139.
Unreal tournament 3: Renders at 290fps. Monitor refreshes at 139.
And NO tearing. So my guess is that when you enable fast sync on a g-sync compatible monitor it locks the refresh rate to 139fps so it can easily pick new frames from the buffer or something and still have room to increase the refresh rate if something goes wrong.
Edit2: Fast sync appeared in my control panel after installing the no mans sky game ready driver and plugging in a Gsync monitor. I have a 970gtx. I thought this tech only worked on pascal?