Jitter is usually caused by small fluctuations in frame rates which happens in pretty much every game. With GSYNC the monitor displays the frame as soon as it is rendered, so if frame one takes 16.42 ms to render and the next frame takes 16.01 ms to render the motion will still be smooth. With a normal monitor you would get a slight jitter.
The only jitters GSYNC can't fix are in poorly programmed engines, typically seen in bad console ports.
I've been using GSYNC for over a year (originally with the GSYNC upgrade kit and now with a ROG Swift). If I ever turn GSYNC off I notice jitter immediately. GSYNC is one of those things that once you start using you can never go back.