Calibre issues aside, getting your Gmail to be fetched as an RSS feed requires authentication, which
must be coded with Calibre's advanced mode - not easy for non-programmers such as myself.
Another option that I found is to use
FreeMyFeed, a free service that takes your Gmail authentication information+Gmail RSS feed, and produces an RSS feed from it which does not require authentication to access. It would probably be wise to forward your Gmail messages to a 2nd account and use that RSS feed in order to prevent 3rd party access to your login info.
Haven't tested this with a Kindle, as mine is yet to arrive..