Expected BEGIN_OBJECT but was STRING at line 1 column 1 path

Question

I am trying to parse JSON which looks like:

{
  "coord": {
      "lon": 3.72,
      "lat": 51.05
  },
  "weather": [
      {
          "id": 800,
          "main": "Clear",
          "description": "Sky is Clear",
          "icon": "01d"
      }
  ],
  "base": "cmc stations",
  "main": {
      "temp": 295.33,
      "pressure": 1020,
      "humidity": 43,
      "temp_min": 294.15,
      "temp_max": 296.48
  },
  "wind": {
      "speed": 3.6,
      "deg": 220
  },
  "clouds": {
      "all": 0
  },
  "dt": 1439990536,
  "sys": {
      "type": 3,
      "id": 4839,
      "message": 0.004,
      "country": "BE",
      "sunrise": 1439959100,
      "sunset": 1440010677
  },
  "id": 2797656,
  "name": "Gent",
  "cod": 200
}

To make Java objects, I used a retrofit response with a callback, which worked. After the retrofit part I called it up and used Gson to parse the JSON response to a WeatherData class. To test if it worked properly, I log the cityname.

 restAdapter.getApi().getWeatherFromApi(txtCity.getText().toString(), new Callback<WeatherData>() {
                    @Override
                    public void success(WeatherData weatherData, Response response) {
                        Gson gson = new Gson();
                        WeatherData data = gson.fromJson(response.toString(), WeatherData.class);
                        Log.i(TAG, data.getName());
                    }

But here it's going wrong, when I run the line where I initialize data, I get the following error:

com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was STRING at line 1 column 1 path $ at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:200)

Is something wrong with my POJO classes? I used following WeatherData class:

public class WeatherData {

    @SerializedName("name")
    private String name;

    @SerializedName("main")
    private Main main;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Main getMain() {
        return main;
    }

    public void setMain(Main main) {
        this.main = main;
    }
}

For getting the minimum and maximum temperature, I created another POJO, Main.class

public class Main {

    @SerializedName("temp_min")
    private double minTemp;

    @SerializedName("temp_max")
    private double maxTemp;

    public double getMinTemp() {
        return minTemp;
    }

    public void setMinTemp(double minTemp) {
        this.minTemp = minTemp;
    }

    public double getMaxTemp() {
        return maxTemp;
    }

    public void setMaxTemp(double maxTemp) {
        this.maxTemp = maxTemp;
    }
}

I guess the BEGIN_OBJECT is the first { of a JSON String, but I can't fix it. What is wrong?