is it possible to remove "Google Frontend" from google app engine response header?

Consider a reverse proxy in front of your GAE app. The DNS will be to your reverse proxy server.

You can mess with the response headers on the way back out.

This starts to break down the killer advantage of GAE - that it "easily scales on googles infrastructure" - but a single nginx server for example will cope with huge traffic.

Once you have outgrown your single reverse proxy - you will probably be attracting money and attention, and no longer be the one worrying about the proxy.

Nope. DNS also gives you away.