The characters in The Great Gatsby are related on varying levels. Tom Buchanan and Daisy Fay Buchanan are husband and wife. Jordan Baker is a friend of Daisy's, dating back to their childhood days in Louisville. Nick Carraway is Daisy's cousin, although they were not close until he moved to the New York area. Nick had been an acquaintance of Tom's when they were both in college but had not been in contact since that time.
Daisy was my second cousin once removed, and I'd known Tom in college...And so it happened that on a warm windy evening I drove over to East Egg to see two old friends whom I scarcely know at all.
Nick meets Jordan that night at the Buchanan's East Egg mansion.
Jay Gatsby is Nick's next door neighbor by coincidence. Nick's very small and inexpensive rental house is located next to Jay's enormous abode. Jay first meets Daisy when he is a young soldier stationed in Louisville and she is actively involved in entertaining as many of the officers as possible. They reunite during the lavish party at the Gatsby mansion, which is the first time Nick actually meets Gatsby.