You may try something like Gentoo Prefix for Mac OS X, which allows you to bootstrap a Gentoo environment under your home directory (or anywhere else you may want it where you have read/write/execute permissions).
From the description:
Gentoo Prefix runs unprivileged, meaning no root user or rights are
required to use it.
Just a note though - I have never personally gotten this to work (it failed a few times and I lost patience). However, I do know it is actively maintained and bug reports are usually replied to very quickly. The IRC channel is also quite helpful.