a theme of hearsay, or "local gossip"? Mrs Gardiner says she "heard", so we can take it she didn't have first hand knowing in that regard.

However, there seems to be evidence of pride and lack of sociability, coming from Darcy being in contact with Lizzy and her neighbourhood. So this could be taken as first hand by Lizzy but not Mrs Gardiner.

Possibly she just put two and two together, and it does kind of match with Darcy's manner in society, what has so far been observed/heard, and certainly her opinion/recollections would have seemed credible.