Setup Compiz-Fusion on Gentoo took a day for me. Even Gentoo Xeffects Wiki is a good place to start.
But before Compiz, I need XGL. Installing XGL is straightforward and easy compared to Compiz Fusion.
The guide available on Xeffects Wiki itself.
I have my own script that may help everyone who want to try Compiz-Fusion on Gentoo, just run it.
You will have fully functional compiz out of your box (But you still need to setup XGL or AIGLX anyway).
This is link for my script