He was a flop coz he was performing at a certain level which got him a move to BM. And since his move to BM he never reached that level, declining every season. Yes he was good but not up to the expected level.
Though, if I remember correctly, Gotze was suffering from the same problem/disease that Ronaldo of Brazil had to deal with at the latter stage of his career. That is, whatever they ate/eat turns into fat. No matter how healthy or how little. Hence, they looked unfit and picking up injuries regularly.