A Gsync supporting monitor does not support a high vertical frequency of 144Hz or 120Hz and so on in the HDMI port.
Try this
Check if your monitor supports the G-sync function. (34UC89G, 32GK850G and etc)
Check if you PC's graphic card supports Nvidia’s G-sync function.
If the above conditions are met, then you can connect PC and monitor via a display port to change it to the vertical frequency of desired Hz on the PC.
This guide was created for all models, so the images or content may be different from your product.