1)

Boot from a Linux USB drive.

2) 

mkdir /mnt/gentoo

3) 

mount -v -o noatime /dev/sd<YOUR_ROOT_PARTITION> /mnt/gentoo

4) 

mount --types proc /proc /mnt/gentoo/proc
mount --rbind /sys /mnt/gentoo/sys
mount --make-rslave /mnt/gentoo/sys
mount --rbind /dev /mnt/gentoo/dev
mount --make-rslave /mnt/gentoo/dev
mount --bind /run /mnt/gentoo/run
mount --make-slave /mnt/gentoo/run

5) 

chroot /mnt/gentoo /bin/bash
source /etc/profile
export PS1="(chroot) ${PS1}"

6) 

mount -v -o noatime /dev/sd<YOUR_EFI/BOOT_PARTITION> /mnt/gentoo/boot

7) 

grub-install --efi-directory=/boot

8)

Reboot & hope...