I have no experience with Gentoo but I would be very surprised if Gentoo did not want to install a bootloader of some kind and want to install it in the MBR of the disk it is being installed on. You may find during the partitioning section that you get options of where to install the bootloader. Ubuntu installs give us this option. If Gentoo installs its own bootloader into the MBR all you have to do is boot into Ubuntu (this is after all a Ubuntu forum and not a Zorin forum) and run these two commands
The first command tells Grub in Ubuntu to update and detect other OS and the second command writes Grub into the MBR of sda thus overwriting what Gentoo has put in there. With a bit of luck you should get a Grub bootmenu with options for all you OS.
sudo grub-install /dev/sda