Say my gmail account gets an email, how can I make it so that any email that comes to my gmail account automatically forwards to another email "bob@gmail.com" with ADDITIONAL parameters in the body of the email? Do I have to use a php script with IMAP or can I do this with gmail alone?
The only way I know to do this reliably is through code. So SMTP/IMAP to send & receive. In Java you can use the JavaMail api. You'll need to find an equivalent package for PHP.