GSync is not meant for 144Hz monitors.
It's meant to reduce the investment in GPU's and allow for 40FPS to be as fluid as 60. It matches the sync rate of the monitor with the framerate of the graphics card. That eliminates tearing, which results from the monitor's refresh rate being 60Hz and that of the card being between 40 and 60.
FreeSync is AMD's implementation of GSync(GSync came first).
Adaptive sync, on the other hand, switches between 30 and 60 FPS, based on actual framerate. It's 30 if actual FPS is between 30 and 60 and 60 if FPS is greater than 60.