I have installed coreboot on 2 different x4x boards. Both with intel G41 chipset and attached GPU to the PCIe slot (so probably not an issue with missing gpu-bios file because the gpu-bios of the PCIe card is been used).
Which mainboards and GPUs are these? Have you tried using the integrated GPU with libgfxinit?
1. I got a clean ssd and installed linux on it using the OEM bios. The typical graphical Grub output was visable and i was able to change configuration in it by pressing 'e' at boot.
2. I used this installed linux and git cloned coreboot (today), built everything and flashed with flashrom the coreboot image.
3. Turned the computer fully off. Turned it on and grub graphical output is gone.
I have this issue since i can think of it.
How i build coreboot:
make menuconfig, choose mainboard-vendor, mainboard model, press esc and confirm saving. Then build.
If nothing else is changed, this should definitely be bootable.