My company uses the same technique at scale to serve animations into emails, often at thousands of gifs generated per second and streamed to the user. Interestingly, when you stream gifs frame-by-frame, some size optimizations are not available to you but there are plenty that are. (cropping, transparency, etc)
Actually, that's not true. GMail proxies images, but doesn't do a lot in the way of caching. They actually cache the DOM when moving between messages for a short while, but a browser reload will reset it.
Been working for 3 years in the past in an email isp I had different information, gmail did cache the images and did it "forever". May be this changed recently?
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...