I don't have complete step-by-step instructions, but it
can be done.
To begin, I recommend you read
my pages on EFI boot loaders for Linux, and in particular the pages on
EFI boot loader principles and
EFI boot loader installation. That will give you an understanding of what happens when you boot in EFI mode, and therefore a fighting chance at creating your own 32-bit EFI-bootable installation medium.
With that knowledge in hand, you can create a regular Linux boot disk (a USB flash drive will be much easier to work with than a CD-R) and add your choice of EFI boot loader or boot manager as EFI/BOOT/bootia32.efi on that medium. You'll then need to "translate" the boot entry for whatever the BIOS boot loader (probably SYSLINUX) does into terms that your chosen EFI boot loader/manager understands. This will entail reading the original configuration file, extracting the kernel boot options from it, and creating a new configuration file for your EFI boot loader/manager that passes those same options to the kernel.
(Note that I'm referring to the "boot loader/manager" under EFI because you can choose to use either a boot loader, such as ELILO or SYSLINUX; or a boot manager, such as rEFInd or gummiboot. If the latter, you'll be relying on the EFI stub loader, which is a boot loader built into the kernel itself. GRUB is both a boot loader and a boot manager.)
With all this in place, your modified Linux installation medium should boot and get you up and running. When you install Gentoo, you'll need to install a boot loader or boot manager to your computer's ESP, much as you would when installing on an EFI 64-bit computer -- but change "x64" in any filenames to "ia32" for your 32-bit platform. Also, if your computer uses Secure Boot, you
must disable it, because the Secure Boot workaround programs for Linux don't compile for x86/IA32, just for x86-64/x64/AMD64.