The command: umount /mnt/gentoo/boot /mnt/gentoo/proc /mnt/gentoo should read: umount /mnt/gentoo/boot /mnt/gentoo/proc /mnt/gentoo/dev /mnt/gentoo
IAfaik the installation doesn't use devfs on /mnt/gentoo/dev, only on /dev. The /mnt/gentoo/dev directory is filled with the necessary device nodes which are handmade (mknod). You shouldn't have to unmount /mnt/gentoo/dev. Did you by any chance bind-mount /dev? Ala "mount -o bind /dev /mnt/gentoo/dev"?