ok it means that you do not have grub installed in the bootsector
boot with the live CD again, mount the gentoo partition somewhere (say /mnt/gentoo)
make sure grub is installed ("emerge -s grub" to see)
if it's installed you'll have a /boot/grub directory
open grub.conf or just copy grub.conf.sample to grub.conf
and make sure something like this is in grub.conf:
#Start of grub.conf
kernel <location of the kernel, eg /boot/vmlinuz-2.4.20> root=/dev/hdX
#End of grub.conf
change /dev/hdX to where your root filesystem is, type "mount" to find out
save the changes
now type "grub"
you'll get the same grub prompt you've been using, first do root (hdX,X) again
this should install grub into the bootsector
Now boot with the bootdisk and use the chainloader command and try to boot it, if it works then repeat the process of extracting the bootsector with dd and update boot.ini.
By the way you don't have a separate /boot partition do you? if so then you should use it instead when setting up the root in grub