sealed class Parent {
  data class Child1(val name: String): Parent()
  data class Child2(val name: String): Parent()
}

class ParentSerializer : JsonSerializer<Parent> {
  companion object {
    const val CLASSNAME = "CLASSNAME"
    const val DATA  = "DATA"
  }

  override fun serialize(
      src: Parent,
      typeOfSrc: Type,
      context: JsonSerializationContext
  ): JsonElement {
    val jsonObject = JsonObject()
    jsonObject.addProperty(CLASSNAME, src.javaClass.name)
    jsonObject.add(DATA, context.serialize(src))
    return jsonObject
  }
}

val gson = GsonBuilder()
        .registerTypeAdapter(Parent::class.java, ParentSerializer())
        .create()

gson.toJson(listOf(Parent.Child1("first"), Parent.Child2("second")))