Not the person you asked, but I use an Acer XB273K GP 27" 4K as my middle/main screen. It does 120Hz just fine, is gsync too.
I also have 2 LG 27UD68-W 27" 4K screens, one on the left in portrait mode, the other to the right in landscape. These were cheap, work fine at 60Hz, but have an incredibly annoying bright standby light that flashes every 2 seconds. There's no way to switch it off, the light comes through the menu joystick on the bottom of the monitor so you can't cover it with electrical tape. And the whole back of the monitor is white plastic, so it glows/reflects some of the LED light. Do not recommend!!
