I think you are saying you want Gentoo's grub to control the boot, and you want its grub in the MBR. But openSUSE installed its own grub to the MBR, and its boot is failing. Is that right? (By the way, you could have instructed openSUSE installer not to do that, and to install grub wherever you wanted, or not at all.)
I presume you do not have a Gentoo CD or DVD to do repairs with. That's OK, you can use the openSUSE DVD. Boot from it into "Rescue System". Login as root. Then do:
Now grub's stage1 is in the sda MBR with a pointer to find its stage2 loader and menu.lst files in sda10.
setup (hd0) (hd0,9)
Keep in mind that Gentoo uses the (less reliable) vanilla grub install script (i.e., "grub-install") while openSUSE uses YaST Boot Loader to feed a script to the grub shell. IIRC Gentoo also uses the "update-grub" script every time there is a kernel update; that rebuilds its menu.lst according to the parameters set for the script in the menu.lst (it is self-documented). Once you are back in Gentoo, you need to run update-grub or edit its menu.lst in order to boot openSUSE.