WhatsApp users can now send and receive their own animated GIFs, or discover and share GIFs from built-in services such as Giphy and Tenor -- which one exactly appears to be dependant on your region.
Likewise, videos that are shorter than six seconds and Live Photos can also be sent as GIFs. To send a Live Photo as a GIF, simply 3D Touch the photo after tapping on Attach > Photo/Video Library and choose "Select as GIF."
WhatsApp is available as a free download on the App Store [Direct Link] for iPhone. The app is now at version 2.16.15.