No, Gsync is intended to eliminate screen tearing. It doesn't serve the same function as a frame rate cap at all.
Let's say you're playing Witcher 3 on a 100hz monitor at 1080p. Your graphics card is humming along, giving you an average FPS of 101, higher than the refresh rate. So what you're looking at on the screen is generally 100fps, and if it drops to 95 once in a while, it's not significant enough to seriously alter your gameplay experience. .
In game, you decide to travel to the Skellige isles. It starts raining in game. The number of npcs on the screen goes up, there's more water texture on screen, more lighting effects needed to be displayed. Your graphics card is no longer giving you 101 fps with occasional drops to 95, now it's giving you 75 to 90 fps on screen, depending on where Geralt is standing at any given moment. Those rates are perfectly playable, and Gsync will have no effect on them, but the jumping around will be noticeable to most people, and really annoying to some. The 75fps moments will feel sluggish.
If instead you capped your FPS at 80, you'd have a smoother gameplay experience. Now, maybe you're the sort of person who isn't really bothered by frame rate variance and you don't want to do that. But I think most people will.
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