I wanted to send a JSON document in form of a JArray (serialized List) from Unity to Cloud Code.

Unfortunately, the method.

var list = new List<int>(){1,2};
var jsonString = JsonConvert.Serialize(list);
var req = GSRequestData(jsonString);

    returns an empty object  

{}

One workaround was to nest the List into another object.

var jObj = new JObject();
jObj.Add("List", lis);

var req = new GSRequestData(jObj);

 results in the correct output

{"List":[1,2]}

I would suggest that you either change GSRequestData or add GSRequestDataList.

It would be great to actually receive an answer from the support.