Wordpress - Enable Gutenberg on custom post type

For Gutenberg to work in a Custom Post Type you need to enable both the editor in supports (which you already have) and show_in_rest. So add 'show_in_rest' => true, to your post registration arguments array.

Start by registering a Gutenberg WordPress custom type. The process is fairly easy and involves adding the following the code snippet.

/*Register WordPress  Gutenberg CPT */
function cw_post_type() {

    register_post_type( 'portfolio',
        // WordPress CPT Options Start
        array(
            'labels' => array(
                'name' => __( 'Portfolio' ),
                'singular_name' => __( 'Portfolio' )
            ),
            'has_archive' => true,
            'public' => true,
            'rewrite' => array('slug' => 'portfolio'),
            'show_in_rest' => true,
            'supports' => array('editor')
        )
    );
}

add_action( 'init', 'cw_post_type' );

add the show_in_rest key and set it to true via your custom post type.

'show_in_rest' => true,
   'supports' => array('editor')

As you can see, the above code snippet just set the ‘show_in_rest’ parameter to ‘TRUE’. After this step, when you create or edit a custom post type, you will see the Gutenberg editor visible and enabled.

All the steps and query discuss in detailed at https://www.cloudways.com/blog/gutenberg-wordpress-custom-post-type/