The main source of jealousy in the novel is between Gatsby, Daisy and Tom. Gatsby is jealous of Tom and Daisy because he loves Daisy greatly. She and him were very much in love before he went to the war. For various reasons she could not marry him and decided to marry Tom instead who was very rich. This makes Gatsby very mad because now he has money and he wants her to come back to him.
He and Daisy had been seeing eachother after they were reunited by Nick and Jordan. Even though they are together in a way Jay is still jealous of Tom and wants to tell him that she is gonna leave him. He seems very uncomfortable when he is around Daisy and Tom. When he sees their daughter al his jealousy rises because she reminds him that Daisy is with Tom.
There is also a small second instance of jealousy between Mr. and Mrs. Wilson. Later in the story Mr. Wilson discovers that his wife has some sort of life away from him. In his jealosy he decides to lock her up in the second story over his garage. He also tells Tom that they are planning to move out west, this is an effort to get her away from whoever she is seeing. He was so jealous that he wants to leave his business and his life.