Dont know if it will solve your issue but:
- your ORGANIZER property value should be a uri, so mailto:foo@bar.com instead of just foo@bar.com
- your are missing the ATTENDEE property which is mandatory in an iTIP Request. See http://tools.ietf.org/search/rfc5546#section-3.2.2
As far as the MIME structure, and seems you seem to care about GMail, the best is to generate an invite from Google Calendar to a non google account, and look at the MIME structure that it generates.