[pablobm]

GMail does a pretty good job at blocking trackers. However, this is not enough to prevent users from triggering the tracking themselves.

As gyardley says, there's the option to enable images. I rarely enable them, but sometimes I decide that I gain more from seeing them than from avoiding the tracking.

Another mechanism is links: clicking on one will get you tracked too. Normally, if there's something that catches my eye I look it up on Google instead of clicking on the link. But sometimes I either don't care or don't see a benefit to do it.

Now, the above is my behaviour, which is that of a paranoid, tech-savvy, power user who, for example, wasn't surprised in the slightest when the whole Edward Snowden story came about. Think of how a normal user will behave instead.