To configure Gmail follow the instructions on this Gmail page: Configuring other mail clients.  A screenshot is also included below - click to enlarge.

NOTE:

1. You must remember to set the Email delivery mode to "Use the settings above to send emails".
2. Check the new settings are working - Click Start > New > Email in Workbooks and send yourself an email.  Look at the Sent Items folder in your Gmail account.  The email should appear there.  If the email has not been received click Start > Emails > Outbound Emails in Workbooks.  If there's an error the email will be highlighted in red.  Click to open the email.  Most errors are caused by configuration typos in email addresses and passwords.