This is what I'd do if I were to add another distro. But this is not gentoo specific, I never used gentoo. So, use it cautiously.
When you get to the installing of boot loader part, tell gentoo not to install one. Instead, tell it to create a floppy. If you can boot gentoo from a floppy and mdk/win from grub, everything is ok.
Also, smart boot manager can help you. Its a piece-of-art boot loader:
http://btmgr.sourceforge.net/
After you did that, and all systems are ok, you can begin to hack your grub until it boots gentoo. Its not difficult to edit menu.lst, and you can't do harm adding entries. The worse thing that can happen is grub failing to boot the new entry, but booting ok the old ones.