Not sure what the problem is, but what version of Intel ME firmware are you on? You should have version 220.127.116.110 for BIOS 2204. Not sure if that could be the cause, but the problem might be that you need to access Windows to change it (although sounds like you can install Windows with the G6900 and then flash it). You can see what ME version you have on the main BIOS screen. If you've tried two i7 CPUs then if it's a hardware fault my money would be on the motherboard. Anyway, do check the ME firmware and make sure the CPU cooler is not overtightened and that the CPU socket has no damaged pins or contains any dirt. Unlikely if the G6900 is working, but worth double checking I think.
Also, just wondering, does the G6900 have fewer contact pads on the bottom of the CPU? If it does, then it might be a damaged pin that is only used for the i7 CPUs. Perhaps the G6900 has the same number of contact pads but some are not used? I don't know anything about the G6900, so nothing more than an idea. Two faulty i7 CPUs does seem highly unlikely.
Z690 Hero, BIOS 2305, ME Firmware 18.104.22.1686, 7000X Case, RM1000x PSU, i9 12900K, ASUS TUF OC 3090TI, 2 x 16GB Corsair RAM @ 5200MHz, Windows 11 Pro 22H2, Corsair H150i Elite AIO, 4x Corsair RGB fans, 3x M.2 NVME drives, 2x SATA SSDs, 2x SATA HDs.