another option, if you don't want to go the CD route, is to boot into single user mode. if you use grub, just hit 'e' when you've selected your gentoo entry, then select the kernel line and hit 'e' again, go to the end of the line, add 'single', hit enter, and hit 'b' to boot it. That should boot you straight to a root-level prompt, where you can change your password. (It's sounds complicated, but it's really not

) It won't change your menu permanently, so next time you reboot, you'll be fine. After you're done, you can exit, and it should continue booting normally.
[If you use lilo, just add 'single' after typing in your label.]