You can find how to recreate your bootloader in the gentoo install instructions - just go to the bootloader section. Cliff's notes:
grub sees your drives as an array indexed from 0, so if /dev/hda is your first hard drive, grub sees it as hd0. If /dev/hdc is your first hard drive, grub also sees it as hd0. If /dev/hdb is a CDROM and /dev/hdc is a hard drive, again, grub sees your hard drive as hd0.
In the same way, grub sees partitions indexed from 0, so /dev/hda1 = /hd0,0
get it? ok....
so, assuming your boot partition is on the first partition of your first hard drive, at the grub prompt, you'd type something like this (grub has tab auto-completion for *everything*)
*note: check the "save" command with auto-complete. It might be "write" or something like that, I forgot.
assuming you still have a grub.conf located in hd0,0 everything should be fine.
if your boot partition is anywhere else, I'm sure you can figure it out, but, say, for instance, your boot partition is on your second drive, third partition, you would do something like this: