Sabayon's installer will do it's best to discover the other operating systems on the disk, but it's not perfect.
So, first, make sure /dev/sda5 is Ubuntu.
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# mkdir /mnt/ubuntu
# mount /dev/sda5 /mnt/ubuntu
Now you can poke around that disk to make sure it's Ubuntu.
You might even want to add an entry for Ubuntu to your /etc/fstab.
If /dev/sda5 is your Ubuntu installation, examine Ubuntu's grub.conf file (should be at /mnt/ubuntu/boot/grub/grub.conf)
It may be as simple as copying the Ubuntu entry from Ubuntu's grub.conf to Sabayon's grub.conf (located at /boot/grub/grub.conf).
BTW, I've run lot's of linux installations from extended partitions. It's never been a problem for me, but YMMV.