If you're using a public IP address as an endpoint, you find the IP and DNS information on the public IP address resource. Or find it in the portal, on the overview page for the application gateway. If you're using internal IP addresses, find the information on the overview page.
For the V1 SKU, gateways created after May 1, 2023, won't have default DNS name automatically associated with the public IP resource.
For the v2 SKU, open the public IP resource and select Configuration. The DNS name label (optional) field is available to configure the DNS name.