Dump this code anywhere in Theme’s template functions.php file (For example, functions.php file for a Theme named “default” would probably reside in the directory wp-content/themes/default/functions.php):
add_filter('gallery_style', create_function( '$css', 'return preg_replace("##s", "", $css);' ) );
And these are the classes for your stylesheet:
.gallery {} .gallery-item {} .gallery-icon {} .gallery-caption {}