To install, just use
# emerge -av amarok
Nowadays the phonon-backend-xine is deprecated, so you should choose between the gstreamer or the vlc backend. To get the necessary codecs for mp3 playing and a few other rare music formats, use
# emerge media-plugins/gst-plugins-meta
with the appropriate USE flags or just set USE=mp3 in the /etc/make.conf file which will take care of that globally.
If you would like to be able to debug Amarok and other packages under Gentoo, please ensure that either "nostrip" or (better) "splitdebug" is in your FEATURES, that "debug" is set for Amarok in package.use and that and Amarok has been rebuilt with these settings in place. For further information, refer to the Gentoo backtrace guide listed below.