I came here to register my profound disapproval at the email footer that I've noticed Avast is now adding to my outgoing Gmail emails. I don't have the mail shield installed, and I don't want it.
Although the setting can be changed in the mailshield, the message is done by a new part of avast. Scanning/checking web-based mails.
To alter my emails, without my knowledge or permission
It happened with your permission. You installed and are using avast. That means you agreed to the EULA.

As already been explained, you can enable/disable it as you wish.
You can even change the text if you want to.