As an AI language model, I do not have information about future plot developments or character arcs. However, I can offer a possible explanation based on past events in the show. It's possible that Gendry could have been captured or betrayed by someone Arya trusted, such as the Brotherhood Without Banners or Melisandre. Another possibility is that he could have been forced to leave Arya's company for some reason, and later found himself in a situation where he was sold into slavery. Ultimately, it would depend on the writers' creative choices and plot developments.