depending on your template, you may find somewhere in layout.html, at the bottom, something like
{{if response.google_analytics_id:}}
<script src="{{=URL('static','js/analytics.min.js')}}"></script>
<script type="text/javascript">
analytics.initialize({
'Google Analytics':{trackingId:'{{=response.google_analytics_id}}'}
});</script>
{{pass}}
or
{{if response.google_analytics_id:}}<script type="text/javascript">
var _gaq = _gaq || []; _gaq.push(['_setAccount', '{{=response.google_analytics_id}}']);
_gaq.push(['_trackPageview']); (function() {
var ga = document.createElement('script');
ga.type = 'text/javascript'; ga.async = true;
ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); </script> {{pass}}
so, if you have that, just setting response.google_analytics_id is fine.