It is probably pointless for Google to use humans to read mails, especially the mails of regular people. And it is certain that they use automated processing of the information in the mails.

A most relevant question would be: Does Google match the profiles of its mail users to high profile people, such as company managers or other rich people who can be easily found on, for example, Linkedin? If so, does Google automatically filter their email in order to predict, for example, the stock market?