Gson gson = new Gson();

Date data = new Date();

String dataNoFormatoJSON = gson.toJson(data);
Date dataDoJSON = gson.fromJson(dataNoFormatoJSON, Date.class);

long dataTime = data.getTime();
long dataDoJSONTime = dataDoJSON.getTime();

System.out.println(data + " - " + dataTime);
System.out.println(dataDoJSON + " - " + dataDoJSONTime);

Tue Oct 17 17:02:03 GFT 2017 - 1508270523483
Tue Oct 17 17:02:03 GFT 2017 - 1508270523000