I used this method to redirect google docs forms to redirect to custom page. (source : morning.am)
<script type="text/javascript">var submitted=false;</script>
<iframe name="hidden_iframe" id="hidden_iframe" style="display:none;" onload="if(submitted) {window.location='REDIRECT-URL.html';}"></iframe>
<form action="GOOGLE-SPREADSHEET-LINK" method="post" target="hidden_iframe" onsubmit="submitted=true;">
But I do not want it to redirect to any page. Instead show a confirmation message in a popup div (like Magnific Popup - inline).