"The problem is that only acknowledged highborn bastards have geographically- based surnames like “Snow”, “Sand” or “Waters”. Gendry didn’t even know who his father is until the very end, so he in fact never uses a surname. He might have know that he is a bastard but he didn’t know he is highborn. Small folks, bastards or not, don’t use surnames. This is evident since many black brothers who are lowborn don’t have surnames. Gendry lived his whole life calling himself simply “Gendry”, so he made a mistake when he wanted to emphasize that he is no longer a bastard, on the same day that he found out he was a bastard of Robert’s. Also it was established that Gendry is illiterate..."