is this a form that you’ve styled yourself? some good reading here: http://morning.am/tutorials/how-to-style-google-forms/
basically its…
replace
<form action="YOUR-EMBEDDED-GOOGLE-SPREADSHEET-LINK" method="POST">
with
<script type="text/javascript">var submitted=false;</script>
<iframe name="hidden_iframe" id="hidden_iframe" style="display:none;"
onload="if(submitted) {window.location='whateveryourredirectis.html';}"></iframe>
<form action="YOUR-EMBEDDED-GOOGLE-SPREADSHEET-LINK" method="post" target="hidden_iframe"
onsubmit="submitted=true;">