Creating a custom field populate with gravity forms and Gutenberg not working

Hi,

I tried your code here, but nothing appears to be stored in db ;
https://support.metabox.io/topic/select-a-gravity-form-to-assign-to-a-cpt/

print_r($meta); is returning nothing

$post_id = filter_input( INPUT_GET, 'post', FILTER_SANITIZE_NUMBER_INT );is returning nothing

Then, also for $saved_form = rwmb_get_value( .... ); which is NULL

This is how I register the metabox :

    // WA Contact page
    $meta_boxes[] = [
        'title'          => esc_html__( '(WA) Contact', 'waff' ),
        'id'             => 'wa-contact',
        'fields'         => [
            [
                'id'   => $prefix . 'c_title',
                'type' => 'text',
                'name' => esc_html__( 'Title', 'waff' ),
                'placeholder' => esc_html__( 'Get in touch !', 'waff' ),
            ],
            [
                'id'   => $prefix . 'c_content',
                'type' => 'wysiwyg',
                'name' => esc_html__( 'Rich content', 'waff' ),
            ],
        [
                'id' => $prefix . 'c_form',
                'type' => 'gform_select',
                'name' => esc_html__( 'Select contact form', 'waff' ),
        ],
        ],
        'category'       => 'layout',
        'icon'           => 'text-page',
        'description'     => esc_html__( 'Display edito bloc', 'waff' ),
        'keywords'       => ['contact', 'post', 'text'],
        'supports'       => [
            'anchor'          => true,
            'customClassName' => true,
            'align'           => ['wide', 'full'],
        ],
        'render_callback' => 'WaffTwo\Blocks\wa_contact_callback',
        'type'           => 'block',
        'context'        => 'side',
    ];

Thanks a lot,
Best regards,
Wilhem