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

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, whether Windows installed

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.

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 too search -u $UUID. Also tell the version of Ubuntu and Gentoo, number of storage devices, OS installed on each storage device, wether Windows installed

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