Setup & Installation

PINN is an easy OS installation tool coupled with a boot loader. With it you can easily install several OSes on a single SD card, which makes it easier to multi boot on different distributions. Each distribution can be used for a different purpose (LibreELEC for Kodi, RetroArch for gaming, Raspberry Pi OS for general purpose, etc).
It seems that PINN strangely does not allow you custom configuration of the partition of the SD card. Partitions are created according to the chosen OSes you want to install, but you can not reserve more space for a given OS if you want to. Of course, you can use gparted after installation to change it as you want.
Normally the OS boot selection is done via keyboard / mouse when booting. For me, this is not ideal as many RPi systems will not be connected to a mouse or keyboard. However, there is a way to change the boot partition (and thus the selected boot distribution) via a text configuration file. So it's relatively easy to write scripts to change the OS via SSH (it requires SSH support for login in all installed OSes, though).

Gentoo on the Raspberry Pi

It seems Gentoo would mostly work on the RPi, but I did not invest too much time. One of the default image in PINN is Gentoo and it works. It even comes pre-installed with Kodi (but not Retroarch) which is great, although I don't know if .
You will mostly want to reinstall Kodi with the webserver USE flag to include support for remote control via the Kore Android application.