Imo, the best way to parse your JSON response with GSON would be creating classes that "match" your response and then use Gson.fromJson() method.
For example:
class Response {
Map<String, App> descriptor;
// standard getters & setters...
}
class App {
String name;
int age;
String[] messages;
// standard getters & setters...
}
Then just use:
Gson gson = new Gson();
Response response = gson.fromJson(yourJson, Response.class);
Where yourJson can be a String, any Reader, a JsonReader or a JsonElement.
Finally, if you want to access any particular field, you just have to do:
String name = response.getDescriptor().get("app3").getName();
You can always parse the JSON manually as suggested in other answers, but personally I think this approach is clearer, more maintainable in long term and it fits better with the whole idea of JSON.