Update:

I bought this cable: https://www.amazon.com/dp/B08QDV5H4M/?coliid=ICX5L11RI99CJ&colid=34NV65B0UEQWR&psc=1&ref_=lv_ov_lig_dp_it

1. switch to dGPU mode
2. Connect to the right hand side usb-c port. And connect HDM2.1 to my LG C2.
3. Default is 4K60 and I can turn on HDR.
4. At the refresh rate selection, there exists 120/100/60 choices
5. However when I choose 100/120hz, the LG C2 will go black for 5s and then go back still with 4K60

Anyone know how to get full 4K120 through the USB4 port of 2022 G14(6900HS+6700S) and which cable to buy? Much appreciated!

The way it's done on the NUC 11 and 12 devices is via the Thunderbolt 3/4 ports, with an adapter like this one https://www.amazon.com/dp/B08QDV5H4M/

Also, the GFN stream is 4:2:0 anyway.