EDIT: Scrapped the first part of this post now that I actually read up on how G-Sync works. As AnandTech explains, the module holds v-blank until the rendered frame is ready, so in theory the error between the rendering timeline and the displayed frame timeline could be lower than the 144 Hz (or 6.94 ms) peak refresh rate.
Something Much More Important
If G-sync is good enough to the point where I would trade it for 60 Hz v-sync, this has some major performance advantages. For instance a 60 Hz v-sync locked game is limited in GPU utilization as a function of it's variability of frame rate. Say the game's GPU run-time varries to as low as 60% of it's peak time. This means the game will target using 60% of 60 Hz (or about 10 ms/frame). This way on the worst frames, it won't miss v-sync. Perhaps the average during gameplay is 80% utilization of 60 Hz (or about 13.3 ms/frame). Roughly on average 20% of the GPU left unused.
With G-sync, say the lower limit of acceptable FPS for the game is 45 Hz (or 22.2 ms/frame). The game's "60% of the peak time" (or fastest refresh) would be 13.3 ms (or 75 Hz). So now the same game runs 45 Hz to 75 Hz refresh rate and uses 20% more of the same GPU on average. That is a big win.
Something Much More Important
If G-sync is good enough to the point where I would trade it for 60 Hz v-sync, this has some major performance advantages. For instance a 60 Hz v-sync locked game is limited in GPU utilization as a function of it's variability of frame rate. Say the game's GPU run-time varries to as low as 60% of it's peak time. This means the game will target using 60% of 60 Hz (or about 10 ms/frame). This way on the worst frames, it won't miss v-sync. Perhaps the average during gameplay is 80% utilization of 60 Hz (or about 13.3 ms/frame). Roughly on average 20% of the GPU left unused.
With G-sync, say the lower limit of acceptable FPS for the game is 45 Hz (or 22.2 ms/frame). The game's "60% of the peak time" (or fastest refresh) would be 13.3 ms (or 75 Hz). So now the same game runs 45 Hz to 75 Hz refresh rate and uses 20% more of the same GPU on average. That is a big win.