What the thread title said. I could do it with my Dell S2716DG. The hard to get requirement for that is making the monitor be stuck in Lightboost (not ULMB) mode even if you try to disable it. That can happen if you have a 3D Vision 2 kit and play in 3D.
I can't say how to get into that state because I don't know how I did it, and after trying to replicate it I've failed, so I lost this secret G-Sync + ULMB mode.
I randomly decided to try Octodad with G-Sync (I hadn't used G-Sync for some time), so I enabled it in the Nvidia Control Panel (the monitor stayed at ULMB mode in desktop (not Lightboost)) and booted the game. "Hey, wtf? This is still strobing.... but wait! It's strobing at 60Hz!". The game is capped at 60fps and I reached the cap, so I didn't realize then what happened.
I was suspicious, so then I started playing Skyrim. And then I was really amazed. It was running in ULMB and synchronizing with the refresh rate! The image quality was perfect. No crosstalk, no tearing, no double image, no extra stuttering, etc. It was G-Sync but without motion blur. I think it synchronized perfectly down to around 30fps. I don't mind flickering, so 40fps with strobing has nothing wrong for me. Brightness was a bit variable, but not as much as you would expect.
Here is the video proof of that (Skyrim with unlocked fps), where you can see the variable strobing (recorded with my Samsung Galaxy S4 Mini camera at 30fps): https://www.dropbox.com/s/u1mhybhc7u2l2 ... b.mp4?dl=0
Around 90fps when looking at the sky (black bars rolling up in the video), and below 60fps when looking at the scenery (black bars rolling down). At exactly 60fps, the bard don't move in the video.
However, there are some things that make the monitor go into normal G-Sync, like games that force a refresh rate change at startup.
Games I tried that worked: Octodad, Skyrim, Castlevania LoS, The Witcher, The Vanishing of Ethan Carter Redux.
After that, I tried to replicate the unlocking of this mode but failed, because now Lightboost is behaving normal (disabling itself when I choose to only enable it in fullscreen applications). Maybe I shouldn't have done that. I would love if Nvidia let us enable this mode.