Something

# mkdir /mnt/gentoo
# mount /dev/sda1 /mnt/gentoo/
# mount -o bind /dev/ /mnt/gentoo/dev/
# mount -t proc none /mnt/gentoo/proc/
# chroot /mnt/gentoo/ 
chroot: cannot execute /bin/sh: No such file or directory
# chroot /mnt/gentoo/ /bin/bash
chroot: cannot execute /bin/bash: No such file or directory
# ls /mnt/gentoo/bin/ | grep bash
bash
rbash
# ls /mnt/gentoo/
backup      dev         lib         opt         sbin        tmp
bin         etc         lost+found  proc        service     usr
boot        home        mnt         root        sys         var
#