Selector label with app:<appname> does not work on Gateway resource


I just started to explore istio and successfully tried installing istio and playaround with bookinfo sample application. I could access the application externally from browser after applying gateway configuration.

Issue i encountered is when i changed Gateway selector with app: productpage it does not work. Not able to comeup with logical reasoning as Gateway selector field accepts app key as far as i can see in multiple examples quoted by other people. Only istio: ingressgateway worked so far for me. Any hints highly appreciated which can help me conclude the app label usage in Gateway CR configuration.