From the day we launched, Gmail has supported something called https. Https keeps your mail encrypted as it travels between your web browser and our servers. We use https to protect your password every time you log into Gmail, but we don’t use it once you’re in your mail. We’re rolling this feature out to all Gmail and Google Apps users, so check back in your Settings menu if you don’t see it right away. Some products that connect to Gmail, like Google Toolbar, are not yet compatible with https.”]
Source: https://gmail.googleblog.com/2008/07/making-security-easier.html