Only for the US market, outside of that there are lots of mail providers which are instead used. GMail addresses are a scarcity in Europe.
Doesn't gmail load images when the email get to the server and not when the email is opened? (That is at least what some who know more about email than I do say) So the only thing tracking pixel really have shown is if the user uses gmail or not
Nope, the image loads when the email is opened by the unique recipient.