The Great Gtasby Pgs. 37-48

I finished reading the next section of F. Scott Fitzgerald's The Great Gatsby, and a few interesting events have happened. One, Tom Buchanan broke his mistresses nose. She kept repeating Daisy's name, and he slapped her in face. This seemed to fit Tom's personality greatly, a large rich man with regards for no one except for himself, it came as no surprise that he also abuses women. On a lighter note, we finally meet the great Gatsby. This mystery man, who we've been waiting to meet has been revealed. Even Nick didn't know who he was though, at first.  There was a quote in the book, that I found a bit confusing, it reads "I was standing beside his bed and he was sitting up between the sheets, clad in his underwear, with a great portfolio in his hands. (Fitzgerald, 38)." What exactly does this quote mean? Why is the man in his underwear, and why is Nick by his bed? This quote brings up some different questions, and I am struggling to understand what exactly it means.