It was the early warning before City grabbed the lead after six minutes thanks to Gundogan's sixth goal in 10 games.
West Brom remain second-bottom on 11 points, six points from the safety zone. It's the Premier League, welcome, it's so tough for everyone.
Next up for Manchester City is a home tie against Premier League whipping boys, Sheffield United, a game which the Catalan insists he is firmly focussed on, rather than the title race.
With this victory, Manchester City have taken their points tally to 41, one ahead of Manchester United.
"Congratulations to the team, we could not leave our box for the first two or three minutes but the rest of the game was good".
Albion failed to clear on the edge of the box, allowing Gundogan to turn Dara O'Shea and fire past Johnstone.
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Guardiola added: "He has always showed this level, except in the first season because he had an ACL".
Unlike last week's opener against Aston Villa, it's hard to argue that City's second goal of the night at The Hawthorns on Tuesday would have had any real impact on the outcome.
Mahrez turned provider after the interval, with his low ball across the face of the penalty area tapped in from close range by Raheem Sterling in the 57th minute.
Fuelled by a sparkling attacking display, Manchester City's players powered to the top of the Premier League for the first time this season - and even the opposition are stopping to admire their work.
"A few home truths needed to be said, it had to be done now because there's a short period of time before the next game", said Allardyce, who held a dressing-room inquest for nearly an hour after the game.
The Premier League have now acknowledged that on this occasion, there was a mistake, telling BBC Radio Manchester (via the Manchester Evening News) that the flag should not have been lifted until the play had come to an end. "I don't know if I've been on a worse run in 20 years but tonight I've seen a little bit of something and we will start again tomorrow". I can accept defeat but not like that. "There's no pressure from the fans so we have to work out what".