How does Mrs. Gardiner know Mr. Darcy in Pride and Prejudice?

Elizabeth's aunt, Mrs. Gardiner, is from the same area as Mr. Darcy. When the Gardiners are introduced in chapter 25 of the novel, she tells Elizabeth about both Wickham and Darcy. Mrs. Gardiner’s knowledge of Darcy is important because it is the same source of knowledge she has about Wickham, who she warns Elizabeth not to marry because he is poor.
The text tells us that Mrs. Gardiner grew up in the area, and that is the source of her knowledge:

Mrs. Gardiner had seen Pemberley, and known the late Mr. Darcy by character perfectly well. Here consequently was an inexhaustible subject of discourse.

It should be noted that she knows the elder Mr. Darcy, who died before the events of the book. His son is the Darcy that Elizabeth eventually falls in love with in the story. Mrs. Gardiner’s knowledge is an integral part of Elizabeth’s initial impression of Darcy and helps to color her prejudices towards the man.