I have always had a problem with Evolution being installed when emerging gnome on my Gentoo system. Even with "-evo -eds" use flags, it still wanted Evolution.
Evolution is a BIG piece of CRAP, and I don't want it anywhere on my system. So I edited the gnome-base/gnome ebuild, and made all evolution stuff dependant on the "evo" or "eds" use flags, which worked fine.
However, I wanted to know why I can't have Gnome without Evolution, and I started asking around... Turns out that you can emerge gnome-light .... Sweet ;-)