The full size images obviously retain their original aspect ratio.
However, the thumbnails can be changed, but it's not dynamic. You basically have to decide whether you want square, landscape or portrait thumbnails, and that will then apply to all of your images (regardless of their original aspect ratio).
You can change this by changing the thumbnail dimensions in Admin CP > Applications > Gallery Options.
After you've done that you'll need to rebuild the media thumbnails before it will apply to your existing thumbnails.