Hi,
mmpOS itself is linux and for a GPU to not get recognized is most likely misconfiguration in bios or pci early init problem. Try disabling fastboot and secureboot if any of these is enabled. Disable HD audio and set pcie speed to 2.0. Check if you have some legacy/uefi mix and set either all to legacy or all to uefi, make sure when using CSM to set "Other OS". These are the common issues that may cause this behaviour. I'd suggest joining discord/telegram channels for faster response and support.