120Hz:
That will give a smoother DESKTOP experience. The cursor, and dragging applications will feel smoother.
GSYNC at 60Hz:
Hard to compare, but if you cap below 60FPS or the game just runs below 60FPS you'll stay in asynchronous mode and have the advantages of no screen-tear, lag reduction etc.
With 120Hz you need to deal with VSYNC ON (no screen tear if you can produce 120FPS in game) or with VSYNC OFF you get screen tearing. It's harder to hit 120FPS, though you can force on the following in NCP:
"Half Adaptive VSYNC" (that will synch to 60FPS.. NCP-> Manage 3d settings-> add game-> (half adaptive vsync) -> SAVE
(Same as normal 60Hz essentially.. it's for when screen tear is bad, but hitting a solid 120FPS is not practical)
So?
Unless you plan to game a lot, I'd go with the 120Hz advantage of smoother desktop. Again, there's no GSYNC benefit if you don't keep the game below 60FPS via cap or just due to the demands on the GPU or CPU (game settings) though some games on max will run above 60FPS.
Sorry if that's confusing.