This is my first post here though I've been reading topics and threads for years
. I'm on AFFC in my Coronavirus ™ re-read and Brienne is after meeting Gendry. I think George highlighted Gendry being angry and scowling partially to contrast with Renly's easy and happy demeanour as Gendry does serve as sort of a "Ghost of Renly" in Brienne's eyes but we also see two other characters with a first impression of Gendry :Ned finds him to be stubborn as a primary characteristic, to Arya he is known as "The Bull" - he's the strongest and most intimidating of the younger NW recruits, but Gendry as we see him in Brienne's POV is angry and scowling - "eyes brimming with anger and suspicion", beating steel like it was a foe in contrast to it being previously described as an all-engrossing art, scowling even more than usual and pretty much his every interaction with Brienne.
He seems to have gone through more of a personality change since the events of the Red Wedding, Beric's death and Stoneheart taking over the Brotherhood than he did while he was in Harrenhal. And Harrenhal was literal hell.
In my opinion this might be indicative of him being at odds with Lady Stoneheart's methods but it's also likely because of Arya's disappearance. We hear him say "Him" when he thought it was the Hound in a way that was non-descript and almost... ominous? It was probably him or Harwin that was calling for Arya when she was taken by the Hound and the biggest difference in his character is this excess of anger so what do you think were George's reasons for rubbing it in quite so much? And do you think it might aid a coming plot point in Winds; whether it be with Brienne's search, Jaime's meeting with Stoneheart, or the theoretical second Red Wedding (he does seem to be developing a vengeful side with him being more violent that we've seen him so far, and of course, following Lady Stoneheart).