gentoo-overlay/xfce-extra/xfswitch-plugin/metadata.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
<pkgmetadata>
  <herd>xfce</herd>
  <use>
    <flag name='gdm'>Use gdmflexiserver from <pkg>gnome-base/gdm</pkg> instead of <pkg>x11-misc/lightdm</pkg></flag>
  </use>
</pkgmetadata>