GMail absolutely caches images. The cache is primarily for performant delivery of data over mobile networks though and reduced load of repeat same-day viewing, and less for privacy.
While their cache is large, it's not infinite. If what you're interested in is whether a customer engages with an email multiple times over a few days, you'll likely get the pixel hits to confirm it over that time period. Of course, as you imply, you wouldn't be able to collect how many times an hour a single customer has viewed an email sent to them.
While their cache is large, it's not infinite. If what you're interested in is whether a customer engages with an email multiple times over a few days, you'll likely get the pixel hits to confirm it over that time period. Of course, as you imply, you wouldn't be able to collect how many times an hour a single customer has viewed an email sent to them.