https://support.google.com/mail/answer/6603
> The process of selecting and showing personalized ads in Gmail is fully automated. These ads are shown to you based on your online activity while you're signed into Google. We will not scan or read your Gmail messages to show you ads.
Here is an except from my "welcome to Gmail" message, from Google, from June 2004:
"You may also have noticed some text ads or related links to the right of this message. They're placed there in the same way that ads are placed alongside Google search results and, through our AdSense program, on content pages across the web. The matching of ads to content in your Gmail messages is performed entirely by computers; never by people. Because the ads and links are matched to information that is of interest to you, we hope you'll find them relevant and useful."