Raspberry Pi is a series of small single-board computers that are supported by gentoo. Some older models are 32-bit only, other support 32-bit and 64-bit.
There are two main ways of installing gentoo. First: cross-compiling and second: using the Raspberry Pi OS or the gentoo "Minimal Installation CD" as a system to provide a chroot.
Some hardware updates since 2012, 32/64-bit and different methods of installation lead to several 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.
- Raspberry Pi/Installation (the previous Raspberry Pi)
- Raspberry Pi/Quick Install Guide
- Raspberry Pi/Minimal musl+busybox cross building
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 hardware is available in the english Wikipedia.