My knowledge might be outdated but back in 2014? Gmail decided to download and cache images embedded in an email. This meant that a tracking pixel was only downloaded once (before a user even opened an email) and every subsequent time the email was opened, the cached image was served from Gmail's servers, not the image origin server. This meant that tracking pixels became useless for Gmail (web based) users. I'm still under this impression today but maybe something has changed?

Edit: I said the pixel loads before the email is even opened. This is incorrect. The pixel loads the first time an email is read. Subsequent loads come from Gmail's servers.