I just finished up adding some UEFI instructions to the Gentoo handbooks for AMD64 and x86 (I don't know how many systems are still using x86 instead of the AMD64 one, and if those support UEFI, but the instructions are shared and they don't collide). The entire EFI stuff can probably be improved a lot, but basically the things that were added are:

1. boot the system using UEFI already if possible (which is needed for efibootmgr to access the EFI variables). This is not entirely mandatory (as efibootmgr is not mandatory to boot a system) but recommended.
2. use vfat for the /boot/ location, as this now becomes the EFI System Partition.
3. configure the Linux kernel to support EFI stub and EFI variables
4. install the Linux kernel as the bootx64.efi file to boot the system with
5. use efibootmgr to add boot options (if required) and create an EFI boot entry called "Gentoo"

If you find grave errors, please do mention them (either on a talk page on the wiki, as a bug or through IRC) so it is picked up. All developers and trusted contributors on the wiki have access to the files so can edit where needed (but do take care that, if something is edited, that it is either architecture-specific or shared across all architectures - check the page when editing; if it is Handbook:Parts then it is shared, and Handbook:AMD64 is specific for the architecture). And if I'm online I'll of course act on it quickly.

Oh, and no - it is not a bug that there is a (now not used) /dev/sda1 "bios" partition. Due to the differences with the possible installation alternatives, it is easier for us (me) to just document a common partition layout than to try and write everything out (making it just harder for new users to follow the instructions).