Why is Nick telling the story?

Nick is telling the story because the Author wants another person in the story to describe Gatsby in a 3rd person perspective and explain his motions in an objective way. This will make Gatsby more heroish  or any way the author wants Gatsby to be.

Why is Nick "confused and a little disgusted" at the end of the chapter?

Nick was confused and a little disgusted because of Tom and Daisy. Tom has a girl in Newyork and Daisy doesn't even think to run away from the house with her baby.