I have this while walking on horseback. I really mean walking, so no holding down A but just moving the analog stick forward/only pressing W.
I also have a G-sync monitor and while G-sync works great most of the time, sadly, the horseback walking animation is screwed. Try it for yourself. While on Roach, just press W to make Roach walk and not run. The camera will auto-center and the animation will skip every second or so. G-sync can't cover crazy stuff like that. "Normal" framerate drops do indeed get masked by G-sync and it sure does look amazing. I'm really enjoying it in TW3, sadly that horseback walking stutter exists.
Edit: Googled and first hit:
http://forums.cdprojektred.com/threads/37068-Stutter-when-riding-horse-at-walking-speed