"I thought you might be here," she responded absently as I came up. "I remembered you lived next door to----" She held my hand impersonally, as a promise that she'd take care of me in a minute, and gave ear to two girls in twin yellow dresses, who stopped at the foot of the steps.
This reminds me of the movie "The First Time" where main character, Dave Hodgeman, falls spontaneously in love with a girl at a party who is currently in a relationship. She flirts back, leading him on, but does it absently, as she is focused on her boyfriend at the time. In the same way, Nick seems fully invested in Jordan while she addresses him plainly and "impersonally" as if preoccupied with something else.
This quote reminds me of one of my favorite childhood movies/books, Ramona and Beezus, where Ramona becomes sick and her dad offers ways to make her feel better. Due to Ramona's interest in art, her dad rolls out an extra long sheet of paper for them to create a mural on. Just as Ramona's dad rolls out a long sheet of canvas to make Ramona feel better on a bad day, I think Gatsby uses caterers with their long canvases to make himself feel better with the use of his riches to throw these nonsensical, extravagant parties.