There is also the Account Activity, 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.
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.)