To save your PowerPoint as GIF, create your presentation, including any animations, transitions, and visuals you like. Choose File > Save as and select Animated GIF from the file type dropdown.

Launch Microsoft PowerPoint, and open the PPTX file for which you want to use a GIF as the background. Select the appropriate slide. Click the "Design" tab, and locate the "Background" section.

The GIF editor is kind of like a simplified video editor—since a GIF and a video both create the appearance of motion in similar (but identical) ways.