convert arrayList multimap to json string?

I have the following code:

public static void postHttpStream(ArrayListMultimap<String, String> fcmbuildProperties){
    HttpClient client = new HttpClient();
    Gson gson = new Gson();
    System.out.println(fcmbuildProperties);
    String jsonString = gson.toJson(fcmbuildProperties);
    System.out.println(jsonString);
}

where fcmbuildProperties is an ArrayListMultimap. I try to convert that to JSON here: String jsonString = gson.toJson(fcmbuildProperties); But this returns an empty array. What do I need to do instead?

This is the input that fcmbuildProperties contain : {build.name=[test_project], build.timestamp=[1425600727488], build.number=[121]}

I need to convert this to Json. with key/values.