1. The java.util.Properties class isn't available on the client side. It's possible to get a properties file to the client side (e.g. using com.google.gwt.http.client.RequestBuilder), but you'd have to find a way to parse it. So why not use JSON format instead? Or GWT-RPC (which can contain the ids as a Collection or Map, or whatever you need)?
3. Another possibility would be to have the values directly in your GWT client side source code. This would however require a re-compile of your client code, and a reload by the user. If you choose this option, and you still want a Properties file or something, you can use a TextResource to directly include it with the GWT JavaScript file (see
https://developers.google.com/web-toolkit/doc/latest/DevGuideClientBundle#TextResource)