The Gmail Whats New page reveals that each time you send and receive attachments, Gmail automatically scans them for viruses.
How does antivirus scanning of email attachments work?
“If a virus is found in an attachment you’ve received, our system will attempt to remove it, or clean the file, so you can still access the information it contains. If the virus can’t be removed from the file, you won’t be able to download it.
If a virus is found in an attachment you’re trying to send, you won’t be able to send the message until you remove the attachment.”
Now that is another step by google’s gmail to protect yours and your email recipients computers from virus laden attachments by its own antivirus measures.