Would be nice to see the real error, when it goes to grub rescue, is it looking for the root dev?
If you enter /dev/sdx x being the the partition your sabayon is on. and it boots, I have seen this.
An easy cure is to boot the live sabayon disk, click on the installer go through the steps to install.
At some point you will be given the option to re-install the sabayon system, or just grub bootloader.
choose grub and reboot.
Am only guessing at your issue with the info provided, easy enough to try.