We don't know, of course, but it's hard to imagine that Mr. and Mrs. Gardiner weren't aware of Mrs. Bennet's shrotcomings as an instiller of a sense of propriety, and I think they would have done their best, in a quiet way, to make up for the lack, in the lives of her two oldest nieces, anyway.

I've wondered why they didn't do the same for the younger nieces, and I think there are several possible explanations. They may have had the press of their own growing family, they may not have had the same feeling of connection with the younger girls, and/or they may have recognized that the younger girls needed a lot more help than they could provide in the relatively short time they would have had with them.