Yes I have installed Gentoo on a Raspberry Pi 3.
The recipe uses NOOBS and Raspbian for a Dual Boot.
Raspbian is used to do a "normal" install and as a Rescue Disc.
The kernel is raspberry pi foundation.
At present, no 64-bit kernels are available, or 64-bit stage-3. People are working on this, but so far nobody has succeeded. Its also not guaranteed that 64-bit is necessarily automatically "better".
The Instructions are here :
The tutorial gives links to info on Over Clocking, and finishes with a raw but bootable copy of Gentoo. Its quick, but leaves you with hints how to use Git to pull in kernel and source code. I managed to load X Windows, Fluxbox and a Browser. Its much faster that Raspbian.