GIFs
You can upload a GIF file in the same way you upload a JPEG or PNG file – via the upload library.
TOP TIP: When creating or selecting your GIF, make sure that the first frame is visible. This is because some ESPs, mainly Outlook, will not let the GIF play automatically. So it will be stuck on the first frame.

Videos
You cannot add a video into an email, however, you can add an image and link that image to the video. It’s better if the image is a screenshot of the video, even with a play button on the top of it, to entice recipients to click on the image.
Add a link to your image by double clicking on the block. A box will appear for you to add your image URL and Alt Text.