Add multiple images in wordpress custom post type gallery

I have created custom post 'gallery' and now i want to add multiple gallery items. please help how i can do this. Here's my code

//Add custom post type for gallery
function cd_custom_post_gallery(){
    register_post_type('gallery', 
        array(
            'labels' => array(
                            'name' => __( 'Gallery' ),
                            'singular_name' => __( 'Gallery' ),
                            'all_items' => __( 'All Images'),
             ),
            'taxonomies'   => array(
                            'category',
                    ),
            'public' => true,
            'has_archive'=>false,
            'exclude_from_search' => true,
            'rewrite' => array('slug' => 'gallery-item'),
            'supports' => array( 'title', 'thumbnail' ),
            'menu_position' => 9,
            'show_in_admin_bar'   => true,
            'show_in_nav_menus'   => false,
            'publicly_queryable'  => false,
            'query_var'           => false
        )
    );
}
add_action('init', 'cd_custom_post_gallery');


function cd_get_featured_image($post_ID) {
    $post_thumbnail_id = get_post_thumbnail_id($post_ID);
    if ($post_thumbnail_id) {
        $post_thumbnail_img = wp_get_attachment_image_src($post_thumbnail_id, 'featured_preview');
        return $post_thumbnail_img[0];
    }
}

What i will add to add gallery items. Thanks in advance.

As mentioned in previous answers yes you can use ACF Plugin, but if you want to do this without using plugin, Make a custom post type Gallery. Then add custom meta field into it for uploading gallery items. Another solutions is that you can use add media option of wordpress post.