Definitely. Here's my off-the-cuff explanation, and maybe someone with more time/technical knowledge will correct me if necessary.
The "Fast" option basically allows the GPU to build every frame it's able to but throws away those that exceed the refresh rate of the monitor. Since it's not caching frames and displaying them late (as occurs in the case of normal vsync) the input latency is minimized, but it requires you to be running at >= the maximum refresh rate of your monitor at all times to have any significant benefit.
Gsync is designed to elegantly handle the inverse scenario, in which your GPU is not capable of generating frames at a rate that consistently equals or exceeds the hardware refresh rate of a standard, non g-sync monitor. Normally, in this scenario, you would experiencing either tearing (with no vsync) or input latency (with vsync) along with possible stuttering every time the framerate exceeded the refresh rate and/or increased/decreased dramatically. There are decent workarounds to this problem, but they all involve limiting your framerate to the refresh rate of the monitor, which also necessarily limits your input rate. Gsync allows the game to determine the refresh rate of the monitor, and that capability connects the graphics pipeline all the way through from game -> GPU -> Monitor; this allows the hardware to dynamically adapt to the software in a way that maintains near-perfect sync across input and output regardless of the moment-to-moment gameplay strain on the CPU/GPU.
Edit: There was another thread here that had some good discussion about these topics also. It might be
http://neogaf.site/forum/showthread.php?p=150548339 ?