Gmail ya incluye Antivirus
Silenciosamente Google ya ha incluido dentro de la ayuda de GMail información acerca de un Antivirus que revisa los archivos adjuntos al ser enviados y recibidos.

How does anti-virus scanning work?
Each time you send and receive attachments, Gmail automatically scans them for viruses.

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.

En la página de Ayuda e Información en español no encontré el texto, seguramente estará disponbile pronto.