A Gson TypeAdapterFactory which allows serialization of @autovalue types. Apache 2 licensed.

AutoGenTypeAdapterFactory.java

import com.google.gson.Gson;
import com.google.gson.TypeAdapter;
import com.google.gson.TypeAdapterFactory;
import com.google.gson.reflect.TypeToken;

public final class AutoGenTypeAdapterFactory implements TypeAdapterFactory {

    @SuppressWarnings("unchecked")
    @Override
    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
        Class<T> rawType = (Class<T>) type.getRawType();

        AutoGson annotation = rawType.getAnnotation(AutoGson.class);
        // Only deserialize classes decorated with @AutoGson.
        if (annotation == null) {
            return null;
        }

        return (TypeAdapter<T>) gson.getAdapter(annotation.autoClass());
    }
}

AutoGson.java

import com.google.auto.value.AutoValue;
import java.lang.annotation.Retention;
import java.lang.annotation.Target;

import static java.lang.annotation.ElementType.TYPE;
import static java.lang.annotation.RetentionPolicy.RUNTIME;

/**
 * Marks an {@link AutoValue @AutoValue}/{@link AutoParcel @AutoParcel}-annotated type for proper Gson serialization.
 * <p>
 * This annotation is needed because the {@linkplain Retention retention} of {@code @AutoValue}/{@code @AutoParcel}
 * does not allow reflection at runtime.
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface AutoGson {
    // A reference to the Auto*-generated class (e.g. AutoValue_MyClass/AutoParcel_MyClass). This is
    // necessary to handle obfuscation of the class names.
    Class autoClass();
}

Main.java

import auto.parcel.AutoParcel;
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

public class Main {
  public static void main(String... args) {
    Gson gson = new GsonBuilder()
        .registerTypeAdapterFactory(new AutoGenTypeAdapterFactory())
        .create();
    Test inTest = Test.of("John", "Doe", 100);
    System.out.println("IN: " + inTest);
    String json = gson.toJson(inTest);
    System.out.println("JSON: " + json);
    Test outTest = gson.fromJson(json, Test.class);
    System.out.println("OUT: " + outTest);
  }

  @AutoParcel
  @AutoGson(autoClass = AutoParcel_Main_Test.class)
  public abstract static class Test {
    public static Test of(String firstName, String lastName, int age) {
      return new AutoParcel_Main_Test(firstName, lastName, age);
    }

    public abstract String firstName();
    public abstract String lastName();
    public abstract int age();
  }
}

@AutoParcel annotated class with a List member, whose element type is also @AutoParcel annotated, won't work. The List member type will be deserialized as com.google.gson.internal.LinkedTreeMap.
Need to dig deeply later.

https://gist.github.com/Piasy/ebaf701e506b0b3d096d