Ya I did some digging last night and there are lots of complaints about G-Sync not being available until ya hook it up VIA the Display Port. Seems to be an issue with the lower Bandwidth of HDMI vs Display Port.
Looks like your monitor is a little older with displayport version 1.2 being listed in it's specs. I think they are up to 2.0 now. I think you should box up your whole computer and send it to me for further testing.
Here check this out @Sikon. https://www.windowscentral.com/hdmi-vs-displayport-which-better-pc-gaming
Cable length matters quite a bit with displayport. The shorter the cord the better off you are. Signal degradation is a thing with DisplayPort.