Fast-sync allows your game to go as fast as the GPU can "pump out" FPS WITHOUT the tearing like the old V-sync off did. I have a 144 Hz monitor and the fastest I could get before Fast-sync came out was 144 FPS with V-sync on, now I can get 170+ FPS in WOFF UE with Fast-sync and no tearing.

G-sync's main benefit is low FPS and Fast-sync benefit is with high FPS, so both can be used at the same time without conflicting with each other.