If you want to run a music server from your Gentoo box and have it show up as a Shared Music in iTunes then follow these steps.
First you need to unmask mt-daapd 0.2.3-r2 os edit /etc/portage/package.keywords and add
Save that and then go ahead and emerge media-sound/mt-daapd, that should install everything that you need to get your music server up and running. I use the ~x86 version because the normal version does NOT seem to work with my iTunes on my mac. You might find different results. All that is left to do is edit the config file, start the service, and add it to boot.
First edit the config to reflect the correct settings, it is located in /etc/mt-daapd.conf It should look something like this but with your values added to it.
servername Media Center
#Shared Music Password
then just start the server and add it to system startup.
rc-update add mt-daapd default
If everything is working you should be able to access the web-based mt-daapd admin page by opening your browser and going to http://localhost:3689 When you open up iTunes you should see your new shared music directory.