Dembele will face his old club in the Champions League tonight.
Gomez, formerly of Barca, was asked about Dembele's problems at the Nou Camp.
He told Sport: "Dembélé is a player who has given a lot to Borussia. With (Thomas) Tuchel, he learned a lot. And thanks to that, he is now in Barça.
"Why does it not work in Barça? These are different championships. I do not know what problem Dembélé has. But he is a very good player, young and I am sure that in the years to come, he will be one of the greatest."