One really neat thing about Grub is that it supports file name completion on the command line using the <tab> key on your keyboard. In this case Grub simply can't find the file you specified as kernel on the command line. When Grub boots and your option is highlighted, press the E key for Edit. Edit the "kernel" line (hit E when that line is highlighted).
Now, you say /dev/hda2 (hd0,1) is mounted at /boot...meaning that /boot actually is the /dev/hda2 partition and under /dev/hda2 there is not /boot directory!
So the first thing I would try is to change the kernel line from:
kernel (hd0,1)/boot/kernel-2.4.20-gentoo-r6 root=/dev/hda5
kernel (hd0,1)/kernel-2.4.20-gentoo-r6 root=/dev/hda5
If that still doesn't work you can use the tab key on your keyboard to find the correct filename - edit the kernel line again and delete the filename and press tab instead:
kernel (hd0,1)/<press tab here>
A list of choices (files in /boot) will be displayed and you can choose the correct one. Then append root=/dev/hda5 to the kernel line, hit enter and then hit B to boot the configuration you just specified.
When you know how it works and you have booted into Gentoo successfully, edit the Grub configuration file /boot/grub/grub.conf and edit it so it matches the configuration you used to succesfully boot. You will have to mount the boot partition first if you followed the Gentoo instructions - mount /boot
will do that - otherwise you won't see anything in /boot.