Samsung Galaxy S8/S8+ and Note 8 updated with GIF support for Always On Display
However, it appears that the GIF animations will only run after locking the phone and only briefly before going still. If you want to animate the image again, you must double tap the screen and the GIF animation should play again.
In order to enable the new feature, after the update head to Settings / Lock screen and security / Always On Display and choose Clock or Home button and clock. You'll then have to return to the Lock screen and security and tap Clock and FaceWidgets option. Then, select Clock style to be presented with a range of clock styles that you can choose from. The one you need to select is shown in the picture to the right of this article.
After you select the correct clock, you will be able to select a GIF file from the phone's gallery or choose one the of default options. Don't forget to tap Apply after you've made all the necessary settings.