When I starting using Gmail I was surprised that the service would not make use of a secure connection between my computer and the service which was a pressing matter because I was using several other Google services that were requiring https to connect. It felt strange and out of place that Gmail did not require https.
It was still possible to open the https website manually but Google does not force the use of https. There is however a setting in Gmail that is called Browser Connection (discovered via Sizlopedia) where the user can select to Always Use HTTPS when he connects to Gmail.
I highly recommend to enable that setting to everyone who is using Gmail as a mail client but especially to those users who use additional Google services that already force https usage.
You can reach the configuration menu by clicking on the Settings link in the top right corner of the Gmail account. Scroll down to the very bottom of the menu that is showing up after clicking on the link and check the Always Use HTTPS box.