First, you can start a Sabayon live cd or stick and then with that live system you can install a new Grub2.
For this, you should really know, on which partition is your hdd Sabayon and then mount it!
On this Grub2 Howtohttp://www.dedoimedo.com/computers/grub-2.html
I found the following chapter:
GRUB 2 can be installed even while you are booted in the OS. You do not need a live environment for that. Just execute the grub-install command against the device or the partition you desire.
<target> can be /dev/hda, /dev/sdb, /dev/sdc4, and so forth.
It is important that you pay attention to the output produced by the command. If you have external disks connected at the time you run the command, it will add these disks to the list of mapped devices. There should be no big harm in that, but if some entries are incorrect, remove them.'
So, you might install Grub2 onto a partition.
Maybe this Howto can help you more to realize, what you want to do.
-Linuxfluesterer (I love KDE ...)