Network sniffers are certain softwares that is used to intercept the network traffic to read and analyze the contents of the packets being passed in a network. In most of the offices, network administrator or managers may have access to some network sniffers to analyze the behavior of their employees through the computer network. With the help of network sniffers, one can get to the details of the data being passed through a particular network. In most of the mail service providers have SSL support (https) for login pages. But the mail contents are not encrypted, in order to reduce the overhead on mail servers and also to save bandwidth.
But, Google has introduced a provision for making the entire GMail session through encrypted channel, so that no one in the network can analyze and interpret the mails being shown to the user. This is in fact a great feature of GMail, which make sure that your private, personal and intimate mails are not being read by a man in the middle of the network. To be specifically your boss or network administrator.
In order to make the entire GMail session encrypted, just go the settings page of GMail and scroll to bottom of the General tab and in the Browser connection section select Always use https. And do not forget to click Save Changes button. Done, all your GMail communications are secured.
Also refer the article on basic computer security, if you are interested.