They do work for the inferred purpose here though, assuming Gmail only downloads them when the email is successfully delivered to the mailbox (and thus the address is valid).

IIRC, google limits their opening of remote images if they're unique per email or from less reputable sources.

They also send you an email back saying your email wasn't delivered after a few days or hours, so there's little benefit in using the tracking pixel to determine if an address exists.

Don't know if they fetch images from emails sent to non-existent addresses, but I would if I were to design such a system.