This is really cool! Completely suboptimal but fascinating none the less. Great project and idea :) would be interested in hearing some people's ideas for real applications of this
One can use automatically generated gifs to add things like countdown timers based on the opening time of the email. (i.e. loading time of the image) See for example this post: https://litmus.com/community/learning/27-how-to-add-a-countd...
More info: https://litmus.com/blog/gmail-adds-image-caching-what-you-ne...