A plugin meant to replace poor native WordPress gallery markup with HTML5 and to add some responsive CSS.
This plugin used to be a good feature for WordPress but with 3.9++ HTML5 support can be enabled with the following snippet :
add_theme_support( 'html5', array( 'gallery', 'caption' ) );
And I think it's much better to handle the responsive part from your main stylesheet now. Considering performance above all ! So you can download the plugin but I recommand not to use it with recent versions of WordPress (after 3.9) so basically not to use it because you're supposed to use the latest version of WordPress anytime ^^