The section of the book that you are asking about can be found in chapter 5, and it is probably my favorite chapter in the entire book. Older readers probably read this chapter and chuckle at Gatsby's actions; however, those same readers can probably empathize with the stress of wanting the perfect date with that special someone. That's exactly what this chapter is about. Gatsby is going to meet Daisy for the first time as a very wealthy man, and he wants everything to be perfect.
The entire event starts out with Gatsby convincing Nick to invite Daisy over to Nick's place for some tea. Gatsby's plan is to just happen to be there in order to bump into Daisy by coincidence. Gatsby has built up in his mind that the meeting needs to be perfect. He arranges for Nick's lawn to be cut, he has flowers sent over, and he arranges for a perfect tea service. Unfortunately, when the day of Daisy's visit arrives, the day itself is not what Gatsby had in his mind. The weather is rainy, and Daisy doesn't show up exactly on time. Gatsby is thrown for a loop and starts to panic.
Finally he got up and informed me in an uncertain voice that he was going home.
"Nobody’s coming to tea. It’s too late!" He looked at his watch as if there was some pressing demand on his time elsewhere.
Daisy arrives moments later, but Gatsby is nowhere to be found. He has quickly relocated himself to a different section of the house in order to assume the perfect pose. He's trying really hard to impress Daisy.
Gatsby, his hands still in his pockets, was reclining against the mantelpiece in a strained counterfeit of perfect ease, even of boredom.
Obviously with Gatsby trying that hard, things don't go so smoothly at first. Nick pulls Gatsby aside and more or less tells Gatsby to "man up."
"You’re acting like a little boy," I broke out impatiently. "Not only that but you’re rude. Daisy’s sitting in there all alone."
Nick then leaves the two of them alone for a half hour. When Nick returns, Daisy and Gatsby have really hit it off.
The reason that Gatsby is so nervous about the entire meeting is that he has built up Daisy in his mind as complete perfection. In his eyes, she is the perfect, most beautiful woman to have ever existed. Gatsby has made it his life goal to turn himself into something worthy of her presence. All of his wealth is merely a mechanism for Gatsby to gain access to Daisy. Gatsby believes that if he is successful in winning Daisy's heart, he will achieve happiness. He's nervous because he realizes that his happiness and life goal might come to a crashing halt if Daisy is not won over by his advances.
Gatsby wants to make everything "perfect" for his meeting with Daisy. He arranges for flowers, tea service, and even for the lawn to be mowed exactly right. He is convinced that he will meet his ideal and thus he strives for perfection. When Daisy does not arrive exactly at four o'clock, the agreed time, Gatsby is dejected. Upon her arrival, he sends Nick outside to greet her while he searches for the perfect pose to be struck upon her first sight of him. In the process, Gatsby breaks a wall mounting, trying to get things just perfect.
Gatsby is nervous because he is so close to his dream. His entire reason of being was to win Daisy. As the "Platonic conception of self," Gatsby has given Daisy to be the meaning of his life. His pursuit and desire to win her favor is the sum total of his being. In seeing her for the first time as a man of wealth, he is nervous with his dream being so close. It was one set of circumstances when he encountered Daisy and he was poor. Yet, now he is rich and he feels that he can get her for himself. In this light, he is nervous when he meets her.