I just did a quick test with G-Sync on my Freesync display.

I didn't see any noticeable tearing.

It seems to work even though I do have a VA panel which isn't the snappiest of display technologies.

All in all, this is a real good thing, the amount of displays that now supports G-Sync is much higher now, and you don't have to shell out a lot of money to get adaptive sync with an Nvidia card now.

TLDR, Nvidia should have done this years ago, but better late then never.