You need to first set-up from which email account TR needs to retrieve the emails. We recommend that you set-up a dedicated email address for that purpose.
Go under Preferences (Mac) or Tools (Windows or Linux) / Options / Miscellaneous, Email Tab.
The fields to be set-up are similar to the ones used to set-up a new email account on your email application.
These values are used by the Retrieve Email function which imports all emails from that account into TR. Each email becomes a thought, with the description set to the subject of the email and the notes set to the body of the email. The emails are deleted from the mail server.
The Retrieve Email option is under the File Menu.
The settings for gmail are:
Email server: pop.gmail.com
SSL: Yes [checked]