I actually wrote a little tool to help test this:
<a href="https:&#x2F;&#x2F;github.com&#x2F;kale&#x2F;image-cache-logger" rel="nofollow">https:&#x2F;&#x2F;github.com&#x2F;kale&#x2F;image-cache-logger</a><p>You&#x27;ll see that it in fact does get cached by Gmail, but only for a limited time.