Add a GIF to a message

1. In the area where you type a message, select   Emoji, Gifs, and Stickers.

   • You can get another gallery to choose from by pressing the  Windows key + . (period) on your keyboard.

2. Select GIFs on the toolbar, and type a term into the Search field to find a specific Gif. 

3. Choose an image to add and select   Send.

You can't import Gifs into the gallery, but you may be able to copy and paste Gifs from a 3rd party site.
 

Tip: If your Gif gallery is missing, open Settings>Privacy and toggle Optional connected experiences off and on.  Additionally, try clearing the teams cache.