Installing Gentoo on Raspberry Pi can be easy, can be hard. There are pre-built images like GenPi64, but this is not under development anymore.
Another way to install Gentoo is to download the stage 3 file and install it like installing on normal PCs, except we need to download firmwares from official repositories.
I have written a wiki page to demonstrate how to install Gentoo on a raspberry pi 5, and it only takes 30 minutes.
https://wiki.gentoo.org/wiki/How_to_install_Gentoo_on_Raspberry_Pi_5
For more details, you can always go to Raspberry Pi Install Guide, which is a new page contains installation guides for all raspberry pi versions.