Gmail automatically downloads images ahead of time, so the tracking pixels will have been fetched by Gmail themselves regardless of when the user opens the email.
Similar signal will be there for any email provider or server-side filter that downloads the content for malware inspection.
Pixel trackers are nearly never implemented in-house, because it's basically impossible for you to do your own email. So the tracker is a function of the batteries-included sending email provider. Those guys do that for a living, so they are sophisticated, and filter on the provider download of images.
At least that's what I remember from them announcing the feature. No idea about other providers, and I haven't tested the feature myself.
It is, of course, very possible that Google has heuristics that can catch tracking pixels—in fact, I would go so far as to say that if they chose to, they 100% could, probably tomorrow. But given where Google makes its money, I would not in the least trust them to do that for me.