Robin-
Those types of fonts are often a pain in gentoo. In my experience the menu fonts looking bad are caused by three different things.
1.) DPI issue
2.) CRT monitor
3.) anti-aliasing
(can be adjusted in several places. xorg, kde control center, probably somewhere in a gnome wm, etc.)
There is a gentoo guide
here which covers a lot of topics but doesn't necessarily cover exact specifics.
Quick Tips:
in /etc/make.conf or package specific in /etc/portage/package.use
USE="truetype .... rest of use flags"
then:
emerge -pv corefonts freetype
Hope it helps.