https://wordpress.stackexchange.com/questions/179357/custom-wordpress-gallery-option

.php

/**
 * Set up the new field in the media module.
 *
 * @return void
 */
function additional_gallery_settings() {
  ?>

    <script type="text/html" id="tmpl-custom-gallery-setting">
        <span>Estilo</span>
        <select data-setting="style">
            <option value="default-style">Padrão</option>
            <option value="carousel-style">Carrossel</option>
            <!-- <option value="ie7-style">IE7 Style</option> -->
        </select>
    </script>

    <script type="text/javascript">
        jQuery( document ).ready( function() {
            _.extend( wp.media.gallery.defaults, {
                style: 'default-style'
            } );

            wp.media.view.Settings.Gallery = wp.media.view.Settings.Gallery.extend( {
                template: function( view ) {
                    return wp.media.template( 'gallery-settings' )( view )
                         + wp.media.template( 'custom-gallery-setting' )( view );
                }
            } );
        } );
    </script>

  <?php
}
add_action( 'print_media_templates', 'additional_gallery_settings' );