While your screen is set to 120hz, have you capped your FPS in game at that range as well?  It could spike past that if not capped and create the screen tearing you're seeing.

When you say that the monitor is set to 120hz, did you check the Nvidia control panel? There will be a drop down in there that you need to select 120hz typically it defaults to 60hz.

G-Sync and V-Sync should be used at the same time. Turn V-Sync on too.

There is a lot of misconceptions about what GSync + VSync does when working together... Since this seems to be a topic that comes up often I will soon be posting about it with source links. Stay tuned. 

Correct. Hardly any one takes the time to read the whole thing.... There are some key parts that are worth highlighting in separate post.

Try toggling vsync on or off and testing both ways. In Witcher 3 for example, with a gsync compatible monitor (I use LG's 34 950F model, similar to this larger one.) the game runs smoother with vsync off. I'd likely DDU drivers after that and swap DP cables after playing around with some settings more. (not sure if the monitor has any necessary updates needed as well.)

LG - UltraGear 38" IPS LED UltraWide HD G-SYNC Monitor

Lots of pixels to drive no matter which card you use .

fakum
Model is 38GL950G-B
Selectable default refresh rates are 60/85/100/120/144 without overclocking

Screen tearing occurs due to one of two reasons; Either the frame rate is below or above the refresh rate of the monitor. G-Sync resolves this issue by dynamically changing the refresh rate of the monitor to match that of the frame rate. The limiting factor here is of course, the G-Sync range of the monitor. In your case the G-Sync range is 1Hz to 175Hz, per Nvidia. There really is not a good reason to have your refresh rate be below 144Hz as all you are doing is limiting the capabilities of your monitor and thus the G-Sync range. When operating within the G-Sync range VSync does not operate as it normally does, but it is critical that it be enabled to ensure a smooth game play. However if your frame rate were to start to exceed the refresh rate of the monitor VSync would start to function as it always has and would introduce input lag, the other bad thing that G-Sync was designed to get rid of. In order to prevent this from occurring you would need need to limit the frame rate within the game if possible to a value that is 3 to 5 FPS lower than your refresh rate. 
 
Nvidia Control Panel Settings (this assumes that everything is currently on default settings): 
- Adjust Image Settings With Preview >> Set to "Use The Advanced 3D Image Settings" 
- Manage 3D Settings >> Monitor Technology >> "G-Sync"
- Manage 3D Settings >> OpenGL Rendering GPU >> Select your primary GPU from the drop down
- Manage 3D Settings >> Power Management Mode >> "Prefer Maximum Performance"
- Manage 3D Settings >> Preferred Refresh Rate >> "Highest Available"
- Manage 3D Settings >> Threaded Optimization >> "On"
- Manage 3D Settings >> Vertical Sync >> "On"
- Change Resolution >> Refresh Rate >> 144Hz
- Set Up G-Sync >> "Enable For Full Screen Mode" 
 
 
My thoughts...
- You mentioned above that you are playing the game in windows mode... while I understand the appeal of doing this, this in its-self could be what is causing all of your issues. 
- In say Call Of Duty I am pushing around 120 to 165 FPS depending on the circumstances @  2560 x 1440 resolution. At that resolution that is 3.69 million pixels with your monitor the GPU is having to drive 6.14 million pixels, nearly double. Which means that I would suspect that in a game such as Call Of Duty you would be looking at a ma frame rate of around 60 to 82 FPS, which honestly is not very high. While I realize that you are not playing Call Of Duty my point is this could just be a simple case of too low of a frame rate for the type of game that you are playing and what you are perceiving as taring is actually just the results of a low refresh rate.   
- Micro Stuttering; This may have nothing to do with the monitor at all and you could actually be experiencing micro stuttering... This can be hard to track down especially if you dont have other games that you can play to eliminate that game as being the issue, but assuming that all of the settings are configured correctly within the game and the Nvidia Control Panel and you do not suspect an issue with the monitor this is a real possibility. What CPU do you have? And hard drive?
- Over Clocking; If you have any overclock at all on your PC (RAM, CPU, or GPU) regardless of weather or not you think its stable disable it all and go back to stock settings and see if the issue continues to occur.   
 

UPDATE: a couple of days ago I elected to simply update my GPU driver. I have about 4 hours run time since, with no tearing at this time. I will report back if things change. Thanks all for your assistance. I learned quite a bit just from this.