As for your grub problem search google for a grub HOWTO. Also there are many good docs at www.gentoo.org,
I believe the gentoo installation doc has a section on how to configure grub for multi os boot.
Your X problem can usually be fixed by logging into linux and as root run the command xf86config. When this program is finished it will have created the file /etc/X11/XF86Config where all X settings are stored. If X is acting up try editing this file and make sure the correct sync values for your screen is there. (Check the monitor manual for those specs)