I think that the high energy, high tempo of Dortmund exposes a lot more players fitness, than our style. Gundogan hasn't lost his technique or his passing range, he 'only' lost a lot of dynamism, that's something that you can regain with hard work but that's not something very crucial in our current set up.
His vision and his technique are what we should focus on, and he still has that at high level.
We also have to remember that he was out for 400 days, he completely lost his professional level conditioning, and you don't regain that easily, at least not when you have to play games every weeks.