Idk why they changed it in the show. Gendry was KNIGHTED by Beric, not given to Melisandre!!!! (Heck, Gendry who's shy around prostitutes and a cautious person in the books would not sleep with a random person he just met, especially one who works for a social class he's resentful towards.) Plus the BWB needed a smith anyway.
Here is Arya's kill list, more to come and more to kill