If you want to add a GIF to your emails, simply follow these steps:
Copy the Image address if the GIF is uploaded to your Google Photos, or copy the GIF URL if you are getting it from Giphy.com
Click on "More Rich"
Click on "Insert Image"
Paste the URL.
Make sure you click on "Preview" to test that the Gif works, and send yourself a test email to make sure it loads in the email.
You can watch the full detailed video by clicking the link here.