UPDATED:
I updated the layout by adding position for the masthead specific:
<?php if($this->API->modules('masthead') && ($item_id != $error_item_id) ) : ?>
<div id="gkHeaderMod" class="gk-clearfix">
<jdoc:include type="modules" name="masthead" style="none" />
</div>
<?php endif; ?>
You can see module on article page now. Kindly check.