I dug through this weird shit and...
It seems that GRUB was tried to use the modules left over from the older Gentoo installation. Some modules must have changed since then, hence the error.
I had to completely reinstall the bootloader and put it manually in the EFI boot table. Otherwise it just wouldn't boot. Seems working so far.
Welcome to your niu world ! We are a cute and loving international community Ｏ(≧▽≦)Ｏ !