Generally speaking, it's a hard case to make to cast Edward and Mrs. Gardiner as surrogate parents in Pride and Prejudice, primarily because the concept as we think of it is a modern construction. The concept of the surrogate parent is defined as individuals who stand in for a child's biological parents and have full rights, duties and responsibilities. In earlier centuries, children might be sent to aunts or uncles for help with rearing them; they might be sent at a young age to apprenticeships or away to school; they might be adopted by well-wishers or even family members but there was no system in place for legally substituting for parents in a contemporary surrogate fashion.
The Gardiners did like Jane and Elizabeth the best of the five girls because they were the most reasonable and rational and, moreover, the least silly and pretentious. Mrs. Gardiner did fill the role of confidant to Jane and Elizabeth and in that role give advice and counsel and also help them, especially Elizabeth, see things in a right light.
Having said this, it is possible to read Pride and Prejudice in such a way that the Gardiners appear to be surrogate parents for Jane and Elizabeth; however the caution must be given that the Gardiners did nothing that was out of place or unusual for aunts and uncles and family friends to do in earlier eras. Some examples of surrogate-like behavior on the part of the Gardiners would be taking a heartbroken Jane to London with them; advising Elizabeth to keep her heart from the "danger" of falling in love with Wickham; taking Elizabeth on a tour of the Lake District (which they never get to).
Larger incidents involve cleaning up the aftermath of Mr. Bennett's failure to safeguard Lydia, who throws herself on Wickham and runs off with him, and in revealing to Elizabeth the secret of Mr. Darcy's involvement in said aftermath. Mr. Bennett turns to Mr. and Mrs. Gardiner for help after Lydia's running a muck, which may support the idea that the Gardiners stand in as surrogate parents who are interested in comforting Elizabeth and Jane and safeguarding their futures, which could be devastated by such a family calamity. Additionally, when Elizabeth discovers that Mr. Darcy was involved in rescuing Lydia, she turns immediately to Mrs. Gardiner for information. On the other hand, Elizabeth and Jane can never seek guidance or advice or comfort from Mrs. Bennett because they must be giving her these things when her "nerves" act up.