Add class to Gutenberg Editor for Custom Post Types

This hook should add a single-[post_type] class to the body of the editor page.

add_filter('admin_body_class', function ($classes) { 
        //get current page
        global $pagenow;

        //check if the current page is post.php and if the post parameteris set
        if ( $pagenow ==='post.php' && isset($_GET['post']) ) {
            //get the post type via the post id from the URL
            $postType = get_post_type( $_GET['post']);
            //append the new class
            $classes .= 'single-' . $postType;
        } 
        //next check if this is a new post
        elseif ( $pagenow ==='post-new.php' )  {
            //check if the post_type parameter is set
            if(isset($_GET['post_type'])) {
                //in this case you can get the post_type directly from the URL
                $classes .= 'single-' . urldecode($_GET['post_type']);
            } else {
                //if post_type is not set, a 'post' is being created
                $classes .= 'single-post';
            }


        }
    return $classes;
});