If Mr. Gardiner had said: "Wickham has debts comming out his ear, but don't worry about it.", Mr. Bennet no doubt would have written to Mr. Gardiner demanding to know how these debts were discharged. By down-playing Wickham's debts, Mr. Gardiner hoped that Mr. Bennet wouldn't ask questions (and perhaps exspose Mr. Darcy's hand in the matter).

Also remember that the Gardiners thought that Lizzy was aware of everything that had happened and perhaps they felt it best to leave it up to her and Mr. Darcy what should be told and what kept secret.