Technically it loads all images through a Google proxy[0] which in theory prevents third parties from using images for tracking. The third parties only get a 'hit' when Google pulls the image into their cache, which is not when you open the mail.

Google can still track you, but if you really care about that then you're probably not using Gmail anyway.

[0]: https://gmail.googleblog.com/2013/12/images-now-showing.html