avast doesn't support SSL connections, it can scan encrypted emails (it doesn't know the encryption key). This has been discussed in a number of threads, so a search of the forums could be useful. A search for Stunnel and Gmail would probably be best as this is a regular use of stunnel to connect to Gmail. The link provided by Technical is probably the best place to start.
How should I go about configuring the scanner? Outlook Express keeps complaining about error with "authentication required" when sending messages.
This is not a scanner issue but an OE issue, you need to tick the box in the accounts settings. Yes, you will have to find a way around the SSL issue, but "authentication required" has nothing to do with SSL. Many ISPs insist you logon to your smtp server to send email, especially if you aren't connected to the Internet through their service.