Getting dressed to the standards of a 1920s gentleman was something of an ordeal, involving a ridiculous amount of layers and accessories. It was a warm day, and they were about to go for a fly, but that was no reason not to dress properly.
So by the time he emerged from his room again he was looking much more like the picture of Jay Gatsby that Nick would have been used to in West Egg. A light suit- vest, shirt, collar, braces, waistcoat, tie, tie-pin- but it was what seemed comfortable and natural to him. Denim was hideous, t-shirts made him feel naked and exposed. There was no reason to catch up with fashions. He thought they're own time very stylish.
"All right, are you ready, old sport?" he asked cheerily, placing a hand on Nick's shoulder as he reached him. "Let's have a little fun."