Besides telling the Gardiners that Wickham has been profligate, in Chapter 43 she tells them even more.

Elizabeth here felt herself called on to say something in vindication of his behaviour to Wickham; and therefore gave them to understand, in as guarded a manner as she could, that by what she had heard from his relations in Kent, his actions were capable of a very different construction; and that his character was by no means so faulty, nor Wickham's so amiable, as they had been considered in Hertfordshire. In confirmation of this she related the particulars of all the pecuniary transactions in which they had been connected, without actually naming her authority, but stating it to be such as might be relied on.

The Gardiners know she has a source of information about Wickham that is not in general circulation. They probably assume it is Darcy.