It says at the beginning of the
Configuring the bootloader article in the Handbook that if you are using MBR/BIOS, that you don't need to make any changes in make.conf to GRUB_PLATFORMS:
When using an older BIOS system supporting only MBR partition tables, no additional configuration is needed in order to emerge GRUB:
root #emerge --ask --verbose sys-boot/grub:2
Then it instructs that if you need or want UEFI support, you need to emerge grub with GRUB_PLATFORMS="efi-64" in your make.conf:
note for UEFI users: running the above command will output the enabled GRUB_PLATFORMS values before emerging. When using UEFI capable systems, users will need to ensure GRUB_PLATFORMS="efi-64" is enabled (as it is the case by default). If that is not the case for the setup, GRUB_PLATFORMS="efi-64" will need to be added to the /etc/portage/make.conf file before emerging GRUB2 so that the package will be built with EFI functionality:
If you followed that portion of the handbook and installed grub with GRUB_PLATFORMS set to efi-64, it effectively removes "pc" as an available platform. Effectively, this would make efi-64 the only available platform to your installation of grub. So ultimately if you want to use grub without UEFI, you need to make sure you DO NOT have GRUB_PLATFORMS set in make.conf. And if you ever had GRUB_PLATFORMS set (or currently do), you'll want to rebuild grub (or just to be on the safe side, rebuild world with emerge -auDNv @world) to ensure you do not have efi-64.
The fact that it seems to be trying to use the efi-64 platform without you explicitly specifying it on the command line indicates to me that you may not have "pc" support in your current install of grub.
Also, one word of pedantry that I'm not sure is completely relevant or not because it's been a while since I use UEFI myself and not BIOS/MBR, in the handbook it says that you need to reference your drive WITHOUT the partition reference, and there seems to be some confusion with respect to what is supposed to be referenced:
Next, install the necessary GRUB2 files to the /boot/grub/ directory via the grub-install command. Presuming the first disk (the one where the system boots from) is /dev/sda, one of the following commands will do:
When using BIOS:
root #grub-install /dev/sda
Also, with respect to your partition table generally, if you set up your partitions similar to the way suggested in
Preparing the disks -> Default partitioning scheme, your /boot should be /dev/sda2 and I don't believe you actually do anything with /dev/sda1.