Mr. Gardiner was just evading a direct recital of how Wickham, and Lydia were discovered. He never mentions Darcy's part in his subsequent letters to Mr. Bennet, even when he was asked about the money he had put forth to 'save' Lydia.

As for Mrs. Gardiner, I beileve the only motive for her long explanation to Lizzy, was because she suspected that Lizzy and Darcy were in love with each other. If Lizzy did not already love him or know of Darcy's goodness, she needed to know it- it might help them to get together.