Attachments in Gmail messages you send and receive are automatically scanned for viruses.
What happens when viruses are found
Virus in an email you're sending
If an attachment you're trying to attach is infected with a virus, you'll see a "Virus detected!" error message. You'll see the option to send the email without the attachment.
Virus in an email sent to you
When Gmail finds a known virus attached to an email that's been sent to you, Gmail will reject the message and let the sender know.
If Gmail finds a virus in an attachment on an email that's already in your inbox, you won't be able to download the attachment.
"Gmail virus scanners are temporarily unavailable" error
You might get this error if Gmail can't scan certain files at the moment. Try downloading the attachment later, or you can open the attachment at your own risk.