In addition to the response codes 403 and 502, the Azure Application Gateway now lets you configure company-branded error pages for more response codes - 400, 405, 408, 500, 503, and 504. You can configure these error pages at a global level to apply to all the listeners on your gateway or individually for each listener. The custom error pages you set are displayed to the clients when the Application Gateway generates these response codes. You can host these error page files at any publicly accessible URLs.
Learn more about configuring the error pages for your gateway.