// Paso 1. Conectar
URL url = new URL("http://preaplicaciones.aragon.es/seda_core/rest/getSignatures?" +
        "applicationId=" + applicationId + "&signatures=1,2,5&organismCodes=ORG99999");
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestMethod("GET");
connection.setRequestProperty("Accept", "application/json");
connection.connect();

// Paso 2. Comprobar respuesta correcta
if (connection.getResponseCode() != 200) {
  // Gestionar respuesta incorrecta
}

// Paso 3. Recuperamos la respuesta del servicio en formato String
StringBuilder responseStrBuilder = new StringBuilder();
BufferedReader streamReader = new BufferedReader(
        new InputStreamReader(connection.getInputStream(), "UTF-8"));
String inputStr;
while ((inputStr = streamReader.readLine()) != null) {
  responseStrBuilder.append(inputStr);
}

// Paso 4. Configurar GSON para recuperar las fechas con tiempo UNIX
GsonBuilder builder = new GsonBuilder();
builder.registerTypeAdapter(Date.class, (JsonDeserializer<Date>) (json, typeOfT, context) ->
            new Date(json.getAsJsonPrimitive().getAsLong()));
Gson gson = builder.create();

// Paso 5. Convertir la respuesta a un array del resultado de ResultGetPublicExternalListInfo
GetSignaturesResponse[] result = gson.fromJson(responseStrBuilder.toString(), GetSignaturesResponse[].class);

// Paso 6. Desconectar
connection.disconnect();