Gracechurch street is where Mr. and Mrs Gardiner's London home is located. This is where Jane comes to stay with them after the Gradiners notice that she is upset at losing the prospect of being with Bingley after he decides to spend the entirety of the winter in London, rather than Netherfield. We see later in the novel that Mr. Gardiner's connections in London make him a better candidate for finding Wickham and Lydia.