I have had someone come to my website looking for help on how to copy an Ubuntu kernel over to a Gentoo installation. This is not too hard to do. Let us say the kernel is version 2.6.35-14-generic. Then you will need to copy the /lib/modules/2.6.35-14-generic folder into the /lib/modules folder on your Gentoo installation. Then you will need to copy the vmlinuz-2.6.35-14-generic kernel image and the initrd.img-2.6.35-14-generic files over to the /boot folder on your Gentoo installation. As well as the System.map-2.6.35-14-generic file which also goes into /boot. Then you will need to update the boot loader and the files you have added will be added to the Gentoo boot loader.
I have done this myself and this does work.
Information on the Gentoo bootloader here: gentoo.org/doc/en/handbook/handbook-x86.xml?part=1&chap=10.