This will update the post status from pending to publish or vice versa when approved/dissaproved using GravityView.

GravityView-update-post-status.php

add_action('gravityview/approve_entries/approved','publish_pending_testimonial');
function publish_pending_testimonial($entry_id){

    $entry = RGFormsModel::get_lead($entry_id);
    if ($entry['post_id'] != null){
        $my_post = array(
            'ID'           => $entry['post_id'],
            'post_status'   => 'publish'
        );

        // Update the post into the database
        wp_update_post( $my_post );
    }
}

add_action('gravityview/approve_entries/disapproved','unpublish_pending_testimonial');
function unpublish_pending_testimonial($entry_id){

    $entry = RGFormsModel::get_lead($entry_id);
    if ($entry['post_id'] != null){
        $my_post = array(
            'ID'           => $entry['post_id'],
            'post_status'   => 'pending'
        );

        // Update the post into the database
        wp_update_post( $my_post );
    }
}