Looks like you forgot to copy your kernel to /boot, or else you typed the name wrong. use your Gentoo boot disk to boot the computer, then mount your drives exactly like you did in Chapter 4.e Mounting (exact commands depend on your partition layout).
mount /dev/hda3 /mnt/gentoo
mount /dev/hda1 /mnt/gentoo/boot
Then you should check to see if your kernel bzImage really is where it should be:
Check to see if the file is there, check the spelling, and update your grub.conf if necessary.