Starting Thursday, Gmail will always use an HTTPS connection when you check or send email, according to the official Google blog.
That means that no one can intercept your messages as they go back and forth between your device and Gmail’s servers, irrespective of whether you're using public WiFi or logging in from public computer, phone or tablet.
Additionally, every email message you send or receive, is encrypted while moving internally. This ensures that your messages are safe not only when they move between your device and Gmail's servers, but also as they move between Google's data centers.
The move is seen as Google's effort to ensure that Gmail users does not fall victim to governments' surveillance that's reported to be on the rise. The security boost from using HTTPS, however may sacrifice performance by increasing latency to a certain degree as noted earlier by Google.