giphy

You can use the site giphy.com to generate animated gif files. The site works with video files that you upload. See the option to create a gif file on the home page at the top right. Simply drag a video file (in .mov or .mp4 format) As you can see there are sliding scales which you adjust to determine where the animation of the gif should be begin and how long it should last:

You are given the option to add a caption to the .gif. After clicking on 'Create GIF', you can click on the Advanced tab for the option to download the .gif file you created.

The resulting gif can be inserted into a PowerPoint slide. It will display in animated mode when you enter presentation mode.