If you're using a client your mail is probably delivered to a local smtp server, might bounce around for a while, and is then delivered to gmail server. You don't know that the last step uses ssl/tsl. Even if it does, what about the 3 other servers it touched? Was it logged somewhere? Did the anti-spam store it? Was it even removed from the spool at all?

My point is, email was not designed to be "secure" (i.e., secret), and is not though that way. Therefore ppl do not work very hard to secure email, and one little band-aid doesn't magically make the whole system "secure".

Your mail could turn up in a log file 5 years from now.