I agree to the points about the error by @rMistero. So now the grub boot from mbr and partition boot record but can't load the basic modules because they can't find it on the expected place (/boot/grub) of that partition. So reinstalling grub will fix that and the best solution is to boot to Gentoo the owner OS of grub legacy (Am I right ?), and reinstall grub. 
As you have grub2 available run the following commands 1 by 1 from the grub2 prompt. 

 set UUID=65a90a0d-a651-4215-8552-0c1859ab61dd
 
 search -u $UUID --set=root
 linux /kernel-3.10.7-gentoo root=UUID=$UUID
 boot

In case it doesn't work give the output of above commands and this one `search -u $UUID` and `search -l main`. Also tell the version of Ubuntu and Gentoo, number of storage devices, OS installed on each storage device, wether Windows installed