Most apps use the ImageTools library. It's a memory hog, but it works. Alternatively, you can try to embed a WebBrowser control in your app and have it load the animated gif.
The application Baconography is so far the only WP8 application I've heard of that uses a custom GIF renderer. The application is open-source, but I don't know if their license allows you to re-use the code in your own app. https://github.com/Synergex/Baconography