Explanation:
The paragraph given in question statement is from Chapter Eight. Starting from the line given, Nick has told himself that he couldn’t sleep because he is strangled "between grotesque reality and savage frightening dreams." Nick told that just as rose has an image of its beauty, so is Daisy “grotesque” in the same way. Gatsby has invested her with beauty which is the center of his dream.
Gatsby has lived "too long with a single dream"; without this dream, his life has become bizarre. In this new world, a world without his dream, Gatsby noticed a rose is just a rose and a green light is not more than a green light. So now Gatsby has been forced to grow up and the world seems im-material to him