Well let's look at Jonny Evans. It's been a bit of a joke, but he's half the player gundogan is but went back to them on the back of a good spell at Leicester.
As I said, we've had a laugh about it, but he's actually played reasonably well for them and afforded decent cover for when their first choice defenders were unavailable.
Gundogan on the other hand has had a great season in la liga with Barca, looked great for Germany over the summer, and was widely regarded as one of the best in Europe not 6 months ago.
He's not starting over Bernardo, kdb, foden, or rodri... But if they need a rest, why not if we can get him back for equivalent of nothing?