well like it says: The drive is allready mounted. So just go ahead and extract the tarball.
Remember to change directory to /mnt/gentoo before extracting.
# tar xvjpf /mnt/cdrom/stages/stage3-<subarch>-2005.1-r1.tar.bz2
If you don't know the exact name of your tarball, just cd into stage dir. to check it out.