You will notice the blue words "Check Status" next to an email within the message. This is the email status feature and is designed to help you see how many people have read the given message (if it's a group message this will show stats for all recipients).
It is important to know though that email tracking isn't very reliable.
In the specific case of "Gmail", they recently changed how they display images in their emails. In the past, Gmail would make a request for an image every time an email was opened. But now they temporarily store those images.
But how does this affect us?
Our email marketing software tracks opens by placing an image in each email. When someone views the images in the email, our image-hosting servers receive a request for the pic, and we use this request to track openings for each subscriber.
Gmail's inbox has changed how subscribers have opened our emails, and caching of images now alters the way we track those openings.
In summary, we simply don't get the data that is very accurate. Email servers aren't required to give us this information, and so reporting is vague at best.