Sending email to Gmail requires you to provide a username/password and a custom port number. This will send the output from DCDIAG in the body of the message.
The following code can be used if you have the Send-MailMessage available to you. This script will send a message if the disk space on C: falls below 125GB. This one will prompt for credentials, but the hash could easily be stored in a file. Not totally secure, but slightly better than keeping it in plain text.