Now it’s possible to add GIF on your Always On Display ! Be sure you’re on the last update ! It just needs to be with the resolution “680х500”.
1. Firstly go in your settings, then click on “Lock screen and security“
2. Then click on “Always on Display“
3. Now click on “settings“
4. You’ll see an animated rabbit, select it then just choose the GIF you want
5. You’re done ! Enjoy