Try this:

Gson gson = new Gson();
String jsonInString = "{\"userId\":\"1\",\"userName\":\"Yasir\"}";
User user= gson.fromJson(jsonInString, User.class);

First of all: there are no "gson" objects. There is just JSON data, strings in a specific format, and the Gson tooling allows you to turn JSON strings into java objects (json parsing), and turning java objects into JSON strings. Coming from there, your request boils down to:

Gson gson = new Gson();
String json = gson.toJson(someInstanceOfStaff);

Beyond that, you should acquire a good tutorial and study this topic, like this one here.

You don't need the intermediary String. Serialize your Java object to a JsonElement directly with Gson#toJsonTree(Object). Cast that value to whatever type you expect (JSON object, array, or primitive), perform your removal and invoke its toString() method to retrieve its JSON representation as a String.

For example,

Gson gson = new GsonBuilder().setDateFormat("yyyy-MM-dd' 'HH:mm:ss").create();
// JSON data structure
JsonElement jsonElement = gson.toJsonTree(javaObject);
JsonObject jsonObject = (JsonObject) jsonElement;
// property removal
jsonObject.remove("property");
// serialization to String
String javaObjectString = jsonObject.toString();

You can always use the Gson#toJson overload that accepts a JsonElement to serialize it directly to a stream if you want to skip that last String object creation.