It seems that with the driver 350.12 even the desktop is using Gsync.
Do you also see that on your side?
So it seems that now G-sync is also compatible with window mode.
In that case running the game in borderless mode will not solve your problem.
So if it is for specific games, is that not enough to disable gsync in their profile? Do you want to do that on the fly in game?
It also means that you have your refresh rate set to 120Hz because ULMB is not compatible with 144Hz.
Switching to a borderless window (if supported by the game) could do the trick since g-sync only works in exclusive mode but SLI works better in exclusive mode also.
Setting gsync and ulmb for some reason does not work for profiles.
For example setting gsync on globally and setting ulmb of dwm.exe (windows desktop) results in fullscreen games switching to 144hz fixed refresh rate instead 144hz gsync. Returning to desktop properly sets it to ulmb.
So what we need is an ulmb command because ulmb - gsync based on profiles has been bugged for years now.