you can have both MBR and GPT on the same disk. GRUB explicitly supports it too, if it finds a EE00 partition type, it will parse the disk again as GPT and boot

you can have both MBR and GPT on the same disk. GRUB explicitly supports it too, if it finds a EE00 partition type, it will parse the disk again as GPT and boot normally to boot a second stage of the ESP. A UEFI system will ignore the MBR and use the GPT table to boot directly into the ESP. The Gentoo wiki has a description how to set this up.