retrofit-converters/gson/src/main/java/retrofit2/converter/gson/GsonConverterFactory.java
@SuppressWarnings("ConstantConditions") // Guarding public API nullability.
  public static GsonConverterFactory create(Gson gson) {
    if (gson == null) throw new NullPointerException("gson == null");
    return new GsonConverterFactory(gson);
  }
@SuppressWarnings("ConstantConditions") // Guarding public API nullability.
  public static GsonConverterFactory create(Gson gson) {
    if (gson == null) throw new NullPointerException("gson == null");
    return new GsonConverterFactory(gson);
  }
Create an instance using {@code gson} for conversion. Encoding to JSON and
decoding from JSON (when no charset is specified by a header) will use UTF-8.