gson invoking standard deserialization in custom deserializer

You can do that by implementing custom TypeAdapterFactory for your object (say CustomClass.class) to be deserialized as below.

 public class CustomTypeAdapterFactory implements TypeAdapterFactory {

    public final TypeAdapter create(Gson gson, TypeToken type) {
     return new TypeAdapter() {
            @Override 
            public void write(JsonWriter out, Object value) throws IOException {
                JsonElement tree = delegate.toJsonTree(value);
                //add code for writing object
            }

            @Override 
            public Object read(JsonReader in) throws IOException {
                JsonElement tree = elementAdapter.read(in);
                //Add code for reading object
            }
        };
    }
  }

And then registering it with Gson as

Gson gson = new GsonBuilder().registerTypeAdapter(CustomClass.class,new CustomTypeAdapterFactory()).create();