Pixel Email Tracking not working for Gmail

Question

I am using a 10 x 10 pixel for email opening tracking, which works fine for every mail provider but Gmail.

Here are my Headers:

header("Content-Type: image/png");
header("Cache-Control: no-store, no-cache, must-revalidate, max-age=0");
header("Cache-Control: post-check=0, pre-check=0", false);
header("Pragma: no-cache");
header("Content-Length: 0");