How to decode JSON with unknown field using Gson?

I have JSON similar to this :

{
  "unknown_field": {
    "field1": "str",
    "field2": "str",
    "field3": "str",
    "field4": "str",
    "field5": "str"
  }, ......
}

I created classes to map this json

public class MyModel implements Serializable {
  private int id;
  private HashMap<String, Model1> models;

  // getters and setter for id and models here
}

and class Model1 is a simple class only with String fields.

But it doesn't work.

Edit: the JSON format looks like this:

{
    "1145": {
        "cities_id": "1145",
        "city": "Nawanshahr",
        "city_path": "nawanshahr",
        "region_id": "53",
        "region_district_id": "381",
        "country_id": "0",
        "million": "0",
        "population": null,
        "region_name": "Punjab"
    },
    "1148": {
        "cities_id": "1148",
        "city": "Nimbahera",
        "city_path": "nimbahera",
        "region_id": "54",
        "region_district_id": "528",
        "country_id": "0",
        "million": "0",
        "population": null,
        "region_name": "Rajasthan"
    }, 
    ...
}