I don't think gsync would effect streaming. it's not like that streaming data passes through your monitor before it goes out to the internet
60fps looks so good on a 60hz display because it's one frame per cycle. if you can't hit 60, and you still want smooth gameplay, you need to drop all the way down to 30fps (one frame every two cycles). any inbetween framerates will either produce screen tearing with vsync off, or a slight stutter and 'input lag' with vsync on. 'input lag' is like the devil to competitive PC gamers, so they usually just deal with screen tearing. there's also debate as to whether or not having a framerate higher than your refresh rate (this also causes tearing) is beneficial
tearing, or stutter and input lag: this is the problem gsync seeks to solve. basically it does it by allowing the monitor to have a variable refresh rate. if the game is running at 46fps, the monitor is 46hz. simple enough, but I guess it took some magic to pull it off with flat panel displays (crt's had variable refresh rates). the gsync monitors are also 144hz natively, so they should be able to deal with a wide variety of framerates (not just 30-60)
so far it's only available with DIY kits (that are sold out) and a particular Asus monitor. but soon there should be a bunch of monitors with gsync support. Asus has this 1440 joint coming soon
http://www.asus.com/us/News/xXtX0FNhXQWPrry7
I'm interested, but I'll probably wait for the prices to come down a little