Main.kt

data class NewDataHolder(
        @JsonAdapter(ObservableTypeAdapterFactory::class)
        @SerializedName("data")
        val data: Observable<DataItem>
)

class ObservableTypeAdapterFactory : TypeAdapterFactory {

    override fun <T> create(gson: Gson, type: TypeToken<T>): TypeAdapter<T>? {
        return if (Observable::class.java.isAssignableFrom(type.rawType)) {
            ObservableTypeAdapter<T>(type.type.getObservableParameterType(), gson) as TypeAdapter<T>
        } else null
    }

}

class ObservableTypeAdapter<T>(private val elementType: Type, private val gson: Gson) : TypeAdapter<Observable<T>>() {

    @Throws(IOException::class)
    override fun write(out: JsonWriter, observable: Observable<T>) {
        out.beginArray()
        observable.blockingForEach { gson.toJson(it, elementType, out) }
        out.endArray()
    }

    override fun read(reader: JsonReader): Observable<T> {
        return JsonReaderObservable(elementType, gson, reader)
    }

}

private fun Type.getObservableParameterType(): Type = getTParameterType(Observable::class.java)

private fun Type.getTParameterType(expectedParameterizedType: Type): Type {
    if (expectedParameterizedType == this) {
        return expectedParameterizedType
    }
    if (this is ParameterizedType) {
        if (expectedParameterizedType == rawType) {
            val actualTypeArguments = actualTypeArguments
            if (actualTypeArguments.size == 1) {
                return actualTypeArguments[0]
            }
        }
    }
    throw IllegalArgumentException(toString())
}