My impression is that Gmail prefetches ALL email images, and then serves them to the reader via their CDN. (Checking a random email in my inbox demonstrates this, https://ci3.googleusercontent.com/proxy/...)
As a result, I thought there was no signal for tracking pixels? I might be wrong though
They only know when google fetches the image, which can be any time between you receiving it and opening it. I highly doubt it's on the fly right when you open it.
All Gmail does is proxy the request to hide your IP from the server hosting the image file. Gmail does not change the timing of the request, the URL, or the image file.
As a result, I thought there was no signal for tracking pixels? I might be wrong though