Gsync is completely different to fast sync, they share some things, but in the end, they have quite different pros and cons, for instance, if you let gsync just run at it's maximum framerate (ie 144) you'll created what Tom Peterson was calling "backpressure", which means massive input latency, fast sync doesn't have this issue, making it ideal for games where you really need 200 FPS atleast, such as CSGO.
personally, I'm hyped as fuck for Fastsync, I've been wanting this sort of technology to become a thing for ages.