gentoo-overlay/gnome-base/gnome/metadata.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
<pkgmetadata>
<herd>gnome</herd>
<use>
  <flag name="classic">Install <pkg>gnome-extra/gnome-shell-extensions</pkg>
    for the Gnome Shell Classic mode</flag>
  <flag name="extras">Install additional GNOME applications</flag>
</use>
</pkgmetadata>