GIFs can be added to any piece of Outreach content (Snippets, Templates, and Sequence steps) and are added the same way you'd add a normal image.
How do I add a Gif?
Simply save the GIF image in a .gif format and insert the image via the image icon located at the bottom of the text box:
Are there size limits
The body of a template has to be ~65KB or less. This means your GIF/image has to be small enough so that the GIF plus the body’s text is under 65KB.