Really? Gmail easily failed this test with flying colors when I tried it: the second I opened the test email, gmail fetched the 1px tracking image, revealing my User-Agent (but not my IP address, granted) to the server.
How did you configure your gmail settings to pass the test? I don't think I changed the default.
It shows that with Gmail, you don't need a browser extension to block pixel trackers.
Gmail does that fine by default, just don't display images in the message.