Try these steps:
1. Create your WordArt as required.
2. Then right-click the WordArt object and choose Format WordArt.
3. In the Colors & Lines tab, click the downward pointing arrow next to the Color fill option and choose Fill Effects from the resultant menu.
4. Select the Picture tab in the resultant Fill Effects dialog box and navigate to where you have stored the animated GIF you want to use as a fill.
The animated GIF will be active (animated) while in slide show mode within your WordArt object.