I haven't read the books yet so I can offer a fresh opinion on this. I'm guessing that it took Arya a lot longer to trust him in the books but for me it didn't felt strange at all. Gendry simply knew the truth about her not being a boy so there was no reason for Arya trying to withhold that truth. And her knowing that he had something to do with her fathers death made her so curious that she gave out herself easily. But to me it didn't felt that strange. I actually found the scene itself quite funny.Episode was good but I'm annoyed with they way they've handled the Arya storyline. 10 episodes isn't enough. I'm not saying that it needs to have every detail from the book but it really hurts the character development when everything is so rushed. I had a problem with Arya trusting Gendry so quickly and revealing her identity to him. Now you have Lommy dying and there's no emotional impact on it whatsoever. Oh hey, some kid that was on the screen for like 4 minutes just died. Who cares? The same thing goes for her saving Jagen and the other two. That scene was so poorly choreographed. I didn't feel they were in that much danger with the fire. Also, I'm not too pleased either with how clumsy she was during that fight.
About Lommy you're totally right. From your post I understand he played an emotional bigger role in the books? I didn't even remember the kid's name up until I saw this post. Felt no connection or anything at the moment he died.