Such deseriazers will help you in processing responses from back-ends which return 200 HTTP code all the time.

BadBackendResponseDeserializer.kt

object class BadBackendResponseDeserializer : JsonDeserializer<Any?> {

    // register all adapters you need
    private val gson = GsonBuilder()
            .registerTypeAdapter(Post::class.java, WallPostAdapter)
            .registerTypeAdapter(Attachment::class.java, AttachmentAdapter)
            .registerTypeAdapter(User::class.java, UserAdapter)
            .registerTypeAdapter(Group::class.java, GroupAdapter)
            .create()

    override fun deserialize(json: JsonElement, typeOfT: Type, context: JsonDeserializationContext): Any? {
        val obj = json.asJsonObject
        val resp = obj.get("response")
        val err = obj.getAsJsonObject("error")

        if (err == null && resp != null) { // got response, no error, just deserialize
            return gson.fromJson(resp, typeOfT)
        }

        // handle error otherwise
        val code = err.get("error_code")?.asInt
        val msg = err.get("error_msg")?.asString
      
        throw when (code) {
            5 -> AuthException("Vk error code: 5. Message: $msg")
            else -> GenericException("Vk error code: $code. Message: $msg")
        }
    }
}

fun usageExample() {
    val deserializer = BadBackendResponseDeserializer
    val gson = GsonBuilder() // regster this deserializer for all types you use as Retrofit Call<type parameter>
            .registerTypeAdapter(TypeTokens.Posts, deserializer)
            .registerTypeAdapter(TypeTokens.Users, deserializer)
            .registerTypeAdapter(TypeTokens.Groups, deserializer)
            .registerTypeAdapter(Void::class.java, deserializer)
            .create()
    val retrofit = Retrofit.Builder()
            .baseUrl("https://api.vk.com/method/")
            .addConverterFactory(GsonConverterFactory.create(gson))
            .build() // create a converter factory ^ with your Gson instance
}