Raspberry Pi

The Raspberry Pi is a series of small single-board computers.

These machines are well supported by Gentoo. There are two main ways of installing Gentoo on them:

cross-compiling
using Raspberry Pi OS, or the gentoo "Minimal Installation CD", as a system to provide a chroot.

Some older models are 32-bit only, others support 32-bit and 64-bit. Some hardware updates since 2012, 32/64-bit and different methods of installation lead to several installation instructions.

Installation Instructions

There is no Gentoo ARM(64) Handbook in general since the hardware and therefore the installation process differs much more than installation for e.g. x86 hardware.

There are several options, and users can choose any one of these:

32-bit

64-bit

Pi 3 64 bit install
Pi 4 64 bit install
In place installation (this will largely work for 32-bit too)

64-bit from GenPi64

Yet another alternative for 64-bit models is to use the images from GenPi64 ; this group is following up on Sakaki's excellent work. These can be dd'ed on a flashdrive. Support and discussions for these all take place on their Discord channel, and they have a https://github.com/GenPi64

There is a not-so-end-user-friendly XML that links to current images ; there should be at least three versions:

OpenRC Lite
OpenRC Desktop
SystemD Lite

Go to the bottom of the XML to find the latest images and matching checksum, append the <key> value to the URL to download.

Hardware

32-bit vs. 32/64-bit models

32-bit only: Zero PCB v1.2/v1.3/W/WH, Pi 1 A/A+/B/B+, Pi 2B (<v1.2), Compute Module 1
32-bit and 64-bit: Pi 2B (v1.2), Pi 3 A+/B/B+, Pi 4B, Pi 400, Compute Module 3[+][lite]/4[lite], Pi Zero 2 W
gentoo not available: Pi Pico

There are some ARM SoCs that support 64-bit but no 32-bit. Those weren't used in the Pis.

A very extensive table of Pi hardware is available in the english Wikipedia.