We use the same technology as mass-email-marketing companies to track our Opens. This is achieved by inserting a small image in the message or Tracking Pixel into the message. When the image is downloaded you get an Open.
As a recipient of such a message who does not wish to be tracked you must either block image fetching from the bananatag servers (assuming they are easily identifiable), block images which look like tracking URLs or are smaller than a certain size, or simply block images altogether.
If it's a new sender you haven't conducted an exchange with (haven't sent an email and such), gmail doesn't auto-download images - you have to request it, like "Display Images" and such.
If you do have regular contact with them, in gmail, go to the settings and select "Ask before displaying external content" under External Content. From there, you should have to confirm before you want images auto-previewed.
By the way, thanks for the question; I didn't know this was out there and I updated my settings appropriately.