Agreed - get an Ultra HDMI Certified cable that‘s rated for 48Gbps, 4K @ 120hz. I picked this one up off of Amazon for ~$12 and it is working great.
This is very very likely the cable, try a different one. 4k120/10bit/Gsync needs power and quality on the cable.
I got the 10 feet blue one today. When I play overwatch in windowed full screen or full screen with Gsync the screen flickers to a gray/pixelated image for a second before returning to normal. Happened multiple times. I turned gsync off and it didn't happen once.
With the other cable it used to sometimes freak out at just a normal desktop (no hdr) at 120hz, this cable solved that issue at least.
Other thing I noticed about this screen is that the color shift is pretty big. Even if you're looking at it directly from the middle, you can still see it. Most noticable with gray / white colors (pretty noticable here).
edit: I've got an MSI Ventus GeForce RTX 3080 10GB framerates seemed to be locked to 118 w/ windowed fullscreen and 130 fullscreen (not sure why that happens, i set it in-game to Match Display)