Please see this forum post regarding making galleries responsive.

Your gallery is already responsive. Its dimensions change when the browser window is made narrower and the gallery elements adjust correspondingly (for example, fewer thumbnails are displayed in a narrow browser window).

Please note that the gallery's height does not change when you change the width of the browser window.
It is not standard behavior for a div's height and width to both change when a browser window is resized in only one dimension.
If you want your gallery's aspect ratio to always remain constant, then you would have to implement a solution such as the one in this forum post.