The Daily Star says the club's CEO Hans-Joachim Watzke admitted last week he was unsure where Gundogan's future lies.
Interest is now mounting in the midfielder with the Gunners and United leading the chase.
Arsene Wenger and David Moyes are both huge admirers of the Dortmund ace and are lining up moves to bring him to England.
However reports in Spain suggest Barcelona are eager to bring Gundogan to the Camp Nou in the upcoming transfer window and will rivals the attempts of both Premier League clubs to sign him.
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