Installing GRUB2 in a GPT disk on a partition

Question

I'm using a MacBook Air 2013 and I'm trying to install Gentoo. I already did it several times but I always used other "workarounds" to this problem such as using directly a EFI-stub kernel image.

What I'm trying to achieve is installing GRUB on a PARTITION so I won't scramble the original disk layout of my Macbook.

This is my partition table:

    GPT fdisk (gdisk) version 0.8.8

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.
Disk /dev/sda: 236978176 sectors, 113.0 GiB
Logical sector size: 512 bytes
Disk identifier (GUID): 9DF6BE23-87E0-4602-B71F-7DAD445D5614
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 236978142
Partitions will be aligned on 8-sector boundaries
Total free space is 6 sectors (3.0 KiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1              40          409639   200.0 MiB   EF00  EFI System Partition
   2          409640       176582599   84.0 GiB    AF00  Customer
   3       176582600       177852135   619.9 MiB   AB00  Recovery HD
   4       177852136       178114279   128.0 MiB   EF00  EFI System
   5       178114280       236978142   28.1 GiB    8300  Linux filesystem

The 1st partition is the original ESP of my Mac (where I installed Refind) and I would like to keep it as it is. I tried to place GRUB on the 4th partition (which is vfat) but when I try to install it there I get:

# > grub2-install /dev/sda4    
/usr/sbin/grub2-bios-setup: warning: File system 'fat' doesnt' support embedding.
/usr/sbin/grub2-bios-setup: warning: Embedding is not possible. GRUB can only be installed in this setup by using blocklists. However, blocklists are UNRELIABLE and their use is discouraged..
/usr/sbin/grub2-bios-setup: error: will not proceed with blocklists.

I also tried using

grub2-install --force /dev/sda4

but that did nothing. How can I solve this thing? Thanks!

Solution

I solved by creating a EFI-stub kernel and then, while booted with it, I proceeded to install grub with grub2-install --target=$arch-efi --efi-directory=$esp where $esp is the directory where I mounted my /boot and $arch is the architecture of my CPU (in my case, x86_64. With this procedure, grub is then installed in the $esp and a grubx64.efi is created.

Last step to be taken is the config creation:

grub2-mkconfig -o $esp

And everything's set!

So to sum it up:

1. Create a EFI-STUB kernel and boot it
2. Install grub as usual with: grub2-install --target=$arch-efi --efi-directory=$esp
3. Create the config file for grub: grub2-mkconfig -o $esp
4. Done!