This software allows you to convert .mov, .mp4, .mpg, .mpeg, .wmv and .avi files to animated gifs. You can loop the GIF if you want the to keep playing over and over. You can also adjust the frame rate to modify the speed the GIF plays at. You can see screenshots of the software as well as a sample GIF that shows the quality you can expect.
You can get this app in the Mac App store, just click the Get this APP button.