According to various sources around the net, Google won't disclose
who is providing the Antivirus technology for it’s web based email service, Gmail.
Since it was just begging to be answered, I decided to find out for myself.
Using various malware picked up from “Google Hack” Honeypots, as well as various sources around the net, I cross referenced the Gmail virus alerts with the Virustotal.com database. Using these pieces of malware, I was able to determine through process of elimination that the antivirus provider for Gmail is Sophos, an industry leader. This is my determination based on the data returned to me from Virustotal.com. More testing would verify my claim.