Because versions of Outlook from 2007 to the present use Microsoft Word to render HTML email, GIFs and animated content don't display well. Compatibility issues cause only the first frame of the GIF to display.
To accommodate for this, we recommend that you produce your file so that the first frame contains the important information. When you're designing emails that contain animated content, always plan for how to minimize the impact on the overall campaign if the animated content fails.
It's usually best to edit your animated content before uploading it to MailChimp. Our built-in Photo Editor alters image files in order to edit them, a process not built for animated content, and this can cause GIFs to break.
Check out Litmus' guide for additional helpful information on animated GIFs and which email clients support them.