Answer
According to http://support.google.com/mail/bin/answer.py?hl=en&answer=13287 outgoing mail using smtp.google.com requires SSL (port 465) or TLS (port 587).
This is possible as of GFI WebMonitor 2013 since support for SSL authentication has been introduced.
Previous versions of GFI WebMonitor only supported standard authentication to mail servers on port 25 and therefore cannot make use of smtp.gmail.com to send email notifications.