When the game is outputting more frames than the monitor can refresh, the monitor will show the next frame before the whole last frame was shown. With a 120hz monitor, the entirety of every frame can be shown of those 120 fps. Technically you're seeing more than 60fps but it's not the whole frames (on the 60hz monitor at 120fps).
G-sync 只係將v-sync solves唔到既input lag issue solves埋, 而且功耗唔會落曬gpu咁解
Q: How much performance gain can I expect to see with G-SYNC technology?
A: NVIDIA G-SYNC ensures that every frame rendered by the GPU is displayed (up to the max refresh rate of the monitor). It does not increase the rate at which the GPU renders a frame.