How can I <iframe> Gmail?
You cannot. GMail, like most of Google's other web services, sets the HTTP header:
X-Frame-Options: SAMEORIGINwhich disallows their site from being framed by any site other than GMail itself.
You would be better off using Google API. http://code.google.com/appengine/docs/python/mail/
Although not a solution to your question I believe this is an appropriate alternative.