that is not coming from Gantry "per se" ... that is coming from Joomla.
The Gantry-head tag is using Joomla functions.
I suggest to use NoNumbers "ReReplacer" to change that.
this is how gantry is using that ...
public function displayHead()
{
if (defined('GANTRY_FINALIZED')) return;
//stuff to output that is needed by joomla
echo '<jdoc:include type="head" />';
}