There is also the [Account Activity][1], where you can see your entire Google monthly activity, including Gmail stats.

> For example, my most recent Account Activity report told me that I sent 5 percent more email than the previous month and received 3 percent more.

[Source][2].

> Google Account Activity creates monthly reports on your usage of Google products. Depending on which products you use, you can see _how many emails you've sent, who your most popular contacts are_, what searches you've performed, or what computers you've used to sign in.

(Emphasis by me.)

[Source][3].

[1]: https://www.google.com/settings/activity/
[2]: http://googleblog.blogspot.com/2012/03/giving-you-more-insight-into-your.html
[3]: http://support.google.com/accounts/bin/answer.py?hl=en&answer=2478305