G-Sync vs FreeSync
Nvidia G-Sync and AMD FreeSync eliminate tearing and stutter by changing the display refresh rate to match the frame rates. Youtuber “Battle(non)sense” released an delay analysis video and explained both technologies.

How G-Sync and FreeSync Work

Let’s say that you got the 144 Hz display and your game runs at 64 fps. In this case both G-Sync and FreeSync will change the display refresh rate to 64 Hz. When a frame rate increases to 105, display refresh rate is also increased to 105 Hz.
In other words, the monitor will change its refresh rate to display an image as soon as the graphics can’t render it.
Both Nvidia G-Sync and AMD FreeSync use same technique to fix tearing, the major difference between the two is that g-sync monitors require a special module provided by nvidia which monitor manufacturers have to pay a licensing fee, while AMD is open standard. Which means G-Sync monitors are in general more expensive than AMD FreeSync ones.

Compatibility

G-Sync Monitor requires Nvidia graphic card.
FreeSync Monitor requires AMD graphic card.

G-sync won’t work with AMD graphic card and vice versa.

After Windows 10 Update

Windows 10 Creators Update breaks the technology, so you need to adjust power settings from Balanced to High Performance as seen below. It is explained in the video below why.

G-Sync vs FreeSync High Performance

Video