donewithwoodenteeth:

What would be the point in not doing so once he figured it out? Why would he pretend otherwise once they were alone. He does wait to talk to her about it until there’s no one else around and keeps the secret afterwards. Why should he pretend to not know?

Really, I think Gendry wanted to get to Arya. He seems to be teasing her and trying to rattle her when he reveals that he knows. 

“She’s no use,” Gendry repeated stubbornly. “Her and Hot Pie and Lommy, they’re slowing us down, and they’re going to get us killed. You’re the only one of the bunch who’s good for anything. Even if you are a girl.”

Arya froze in her steps. “I’m not a girl!”
“Yes you are. Do you think I’m as stupid as they are?”
“No, you’re stupider. The Night’s Watch doesn’t take girls, everyone knows that.”

“That’s true. I don’t know why Yoren brought you, but he must have had some reason. You’re still a girl.”

“I am not!”
“Then pull out your cock and take a piss. Go on.”
“I don’t need to take a piss. If I wanted to I could.”
“Liar. You can’t take out your cock because you don’t have one. I never noticed before when there were thirty of us, but you always go off in the woods to make your water. You don’t see Hot Pie doing that, nor me neither. If you’re not a girl, you must be some eunuch.”

“You’re the eunuch.”

“You know I’m not.” Gendry smiled. “You want me to take out my cock and prove it? I don’t have anything to hide…“I would if I knew, Arry… is that really what you’re called, or do you have some girl’s name?” 

Gendry actually seemed to be having fun teasing her. He probably wanted to show her up, too, in the sense prove that he saw through her deception. He wanted to rattle her and did-

Until she inadvertently turns the tables on him by revealing she’s Arya Stark that is.