If your web browser says https:// before the URL on your browser command line.
But it does not encrypt the communication sent from the gmail, or ensure that ONLY the people meant to read your message reads the email.
IMHO, don't commit anything to email you don't want EVERYONE else to read. No excuses, just don't. Figure it is like posting whatever you write on a huge bulletin board that everyone can read with a little effort, not just your intended audience.
Are there ways to get around this? Yes, but using web based services is not the way and it takes considerable effort to ensure private communications from 'end to end'