Gundogan has been linked with Barca and Manchester City.
"In my eyes, a switch to Barcelona would come too early for him," the 47-year-old told Bild. "He is barely 22. He can think about such a move again when he is 25 or 26.
"What is to come of him at Barca? The guys at Borussia really should light a candle everyday. There can be nothing better for this club than to play under [coach] Jurgen Klopp."