File: /etc/gdm/gdm.schemas

<gdmschemafile>
  <schemalist>
    <schema>
        [...]
    </schema>
    <schema>
      <key>security/DisallowTCP</key>
      <signature>b</signature>
      <default>false</default>
    </schema>
  </schemalist>
</gdmschemafile>