Ok I've finally gotten it to load the kernel and boot, by booting it from the Ubuntu GRUB file with
title Gentoo
kernel (hd0,4)/boot/gentoo root=/dev/sda3
After copying my gentoo kernel image to /boot/gentoo on my Ubuntu drive (hd0,4) is Ubuntu drive "/dev/sda3" is my root gentoo partition.
Upon trying this it starts to boot, then says something about having no NFS server and being unable to load sda3, (I can take down the error message if you need it, probably should've done it when it happened first)
Any ideas? If not I'll take down the error message next time I reboot