Add custom setting that uses radio button to WP Gallery

I had the same problem and got it finally to work. Based on the thread you have followed, you now have to extend the “update” function of the WP core /wp-includes/js/media-views.js.

Go to the “jQuery(document).ready(function()…” part of the ‘print_media_templates’ action and try this:

<script>

    jQuery(document).ready(function()
    {
      wp.media.view.Settings.Gallery = wp.media.view.Settings.Gallery.extend({

        // add your custom options template (you probably already did this)
        template: function(view){
          return wp.media.template('gallery-settings')(view)
               + wp.media.template('custom-gallery-setting')(view);
        },

        // now extend the update function 
        update: function( key ) {

           // call originial function: you want to *extend* it, 
           // not to override it completely!
           wp.media.view.Settings.prototype.update.call( this, key );

           // get the current value of your gallery_image_title_text_size attribute
           var currentTextSize = this.model.attributes.gallery_image_title_text_size;

           // get the data-setting elements, in your case 'gallery_image_title_text_size':
           $setting = this.$( '[data-setting="gallery_image_title_text_size"]' );

                    // for each of this setting (if it's a radio button),
                    // check the value stored by your attributes 
                    // and set the radio button accordingly
                    $setting.each( function ( index ) {
                        if ( $( this ).is( 'input[type="radio"]' ) && 
                             currentTextSize == $( this ).val() ) {
                            $( this ).attr( 'checked', true );
                        } else {
                            $( this ).attr( 'checked', false );
                        }

                     } );
            }
        } );
    } );           
</script>