I have a dual-boot system, on /dev/sdb4 I have Gentoo, and on /dev/sdb5 I installed Ubuntu 18.04 afterwards. My boot partition is /dev/sdb2, and I let Ubuntu handle the whole bootloader stuff. And that worked well, Ubuntu found my Gentoo installation, added it to Grub, and I was able to boot into both operating systems.
Now I updated the Kernel within Gentoo, recompiled it and
emerge --depcleaned, so the old Kernel is gone now. Didn't think much about it.
But now I can't boot into Gentoo anymore. There's still a Grub entry for it, but it doesn't work, so Grub boots into Ubuntu nonetheless. I've tried
sudo grub-install /dev/sdb again, and also
update-grub from within Ubuntu. It still says that it finds the Gentoo installation, and there is still a Grub entry for it, but it is not bootable.
I have mounted both Gentoo's and Ubuntu's /boot directory to /dev/sdb2, but let Ubuntu clean it in its own installation, so I fear that Gentoo's /boot directory points to the partition that Ubuntu has erased and created anew.
I'm still on a BIOS-system, by the way. No UEFI yet.
How can I repair this mess without having to go through reinstalling kind of everything?