This article applies to:
Question: https for mail.google.com is slipping through the filter
Make sure you are blocking “Web-Based Email” in the profile, and https filtering level is set to “Medium”.
Go to System>Control>Filter, HTTPS filtering to “Medium” with forward lookup to validate qualified dns checked.
That should handle the issue; you will receive a "page cannot be displayed" instead of the block page, because the filter cannot return a block page on encrypted traffic.
Clear the cache on the browser before testing.