I don't really see it this way. For many people, the only company of consequence that is tracking them is ultimately Google (and/or Facebook). Trackers that other companies install in their emails or websites are just sending the data back to Google in the end anyway. It's a redundant way for Google to capture information to build a profile of you with, but if you're using Gmail anyway, they don't need the extra tracking, they still get the same information.

That’s not true. Many advertisers and even newsletters try to figure out which of their emails you actually read and when, so they can optimize subject and date/time for better effect - e.g. emoji in subjects on Sunday get better hits with person X, finance data on Thursday evening with person Y.

They used to be able to tell where you were reading it from geoip, but google killed that by proxying all images through their servers as of a few years ago.