The issue is your dGPU that doesn’t have Legacy fw image vBios, only has modern EFI firmware vBios for UEFI environment.
Confirm your dGPU firmware and request it to GigaByte.
EDIT:
Oh… and you forgot to mention this… just great!!!
Anyway, if it’s the same for iGPU use UEFI Editor and share here the “newly” Gigabyte bios.