I'm not sure if I can make clear but I'll try😅 You're right it's adding a nuance. The grammar /는 걸(요) / 인 걸(요) is usually used to express the idea of something that is unexpected or maybe even contrary. 갑옷을 입어 본 적이 있나? 없습니다. 입는 법도 모르는 걸요 the nuance here is I don't even know how to wear it so how I could possibly try wearing it. Another example А: 부산 가봤어? B: 아니, 아직 한국 못가봤는 걸 ->nuance here is I haven't been to Korea yet so how can I go to Busan?