There are a few potential issues here:
Firstly,
Deus Ex: Mankind Divided is a very CPU-heavy game.
If the cause of the stuttering is due to CPU load, G-Sync can't fix that. CPU-limited games often have very "spiky" frame-time graphs, while GPU-limited games tend to have nice smooth frame-time graphs.
You may want to try toggling Windows 10's Game Mode on or off and see which runs better. For my system with an R7-1700X, enabling Game Mode gives better minimum framerate numbers, but introduces stuttering. It may be the other way around for a mobile CPU.
Secondly, make sure that you are using the DX11 renderer.
The DX12 renderer performs worse and is more prone to stuttering. Frame-times measure much worse in DX12 for me.
Thirdly, I noticed that sometimes G-Sync was not active when the game was running in borderless mode.
It seemed like it may be linked to starting the game via the launcher,
rather than using the -benchmark launch command to skip it and the intro videos.
When that happened, I had to alt-tab away from the game and back to it for G-Sync to activate. (just once I got to the menus for the first time)
Testing the benchmark on my system; an R7-1700X with a GTX 1070 on a PG348Q, it was actually running perfectly smoothly for me, which was not the case back when I built this system at the beginning of April. The game heavily favors Intel CPUs, and I was very disappointed in its performance back then because I couldn't get it to run smoothly.
A lot has changed since then, so I can't point at any one thing which may have fixed it; AMD updates, NVIDIA drivers, Windows 10 FCU improvements etc. Make sure your OS and drivers are up to date.