The only significant thing i'd add to the discussion on this is a definition and a gotcha (which i don't think occurs in GSync monitors due to the cost of licensing fees)
Adaptive Sync (GSync, FreeSync) is the opposite of VSync.
VSync syncs your video card to your monitor refresh rate, preventing delivery of a new frame until the monitor is ready to accept it. This causes backups in the frame buffer cache, and when the card operates way above the refresh rate, if no other pacing mechanism exist (these days, there are driver pacing mechanism) can result in significant stutter (irregular frame pacing) in the frame buffer, which makes the game appear to be doing something I call breathing.
Adaptive Sync lets the video card control the frame rate. When frames are completed, they are "written" to the monitor. Monitors which support adaptive sync technology have a range of available refresh rates, and can change the rate on a frame by frame basis. This completely prevents tearing (until you exceed the monitor's refresh rate cap) and can make action at very low framerates more playable. Even below 45 fps, so long as the delivery is reasonably smooth (low stutter) the game will still look and feel great.
The "gotcha" is that because Freesync is a royalty free and open standard, while monitors with Freesync can be cheaper, the market space is sullied with budget monitor manufacturers including the tech, even though the range of capable frame rates might be something trivial like 55-75hz. This is in opposition to what you really want, something that goes say, 25hz-144hz.
Pay close attention to the refresh rate range when buying an adaptive sync monitor. As i mentioned already, I doubt this affects GSync due to license fees, but i can't say for sure, as i'm on team Red and have never shopped for a GSync monitor.