Paranoid much?

Google Mail does not scan the content of your emails for the purpose of showing you ads. However, Google Mail does use some information about your emails, such as your email address and the sender and recipient addresses, to improve its spam and phishing filters.

You can control how much information Google Mail collects about your emails by adjusting your privacy settings. For example, you can choose to have Gmail not scan your email attachments for viruses.

Just read their general terms.