Gundogan and Kovacic can play in his position, but maybe Guardiola will have to buy another player in next transfer window.
I didn't watch much of Barcelona last season, so I'm not really sure how he played there, but Gundogan is nowhere near as good as he was before he left City, and it shows. Another player who is underperforming is Walker. He should have been benched, as he has been a liability the entire season. Today's game against Newcastle was a good example of that.
Has there been a game that they won comfortably without him?
I am not sure what you consider a comfortable win but they looked pretty solid few first games of the season with Kovacic playing at his position.