In chapter seven - How does Nick react when he realizes Gatsby's innocence?
When Nick thinks Gatsby was the one who ran over Myrtle, he has intense dislike for him. In fact, he is disgusted with whole crew--Gatsby, Jordan, Daisy, Tom, after the fiasco at the Plaza Hotel. After Gatsby tells Nick that Daisy was the one actually driving, we do not have Nick's initial response. Chapter 7 closes with Gatsby standing outside Daisy's house watching to see if she flicks on the light, signaling that she needs his help. Nick had already glanced into the Buchanans' house and knows that Daisy and Tom have moved on. Daisy would not be needing anyone's help. So, he leaves Gatsby alone.
However in the following chapter, Nick goes over to Gatsby's to say goodbye. Upon leaving, he tells Gatsby, "They're a rotten crowd" and that "You're worth the whole damn bunch put together!" Nick has at this point separated Gatsby from the others, and decided that he is their moral superior. After Gatsby is killed, it is Nick who stays around and takes responsibility for notifying Gatsby's father and "friends." This last act of Nick's is a tribute to Gatsby who according to Nick "turned out all right in the end." He decides that the others are "careless people" who "smashed up things and creatures and then retreated back into their money."
At first, Nick thinks that Gatsby has run over Myrtle and kept on going, without even the decency to stop. Nick therefore feels a dislike that only intensifies when Gatsby speculates that nobody saw his car hit Myrtle:
I disliked him so much by this time that I didn't find it necessary to tell him he was wrong.
But shortly thereafter, Nick "guessed at the truth" that Daisy was driving and asks Gatsby if it is true. Gatsby says yes. Nick finds out too that it is Daisy who kept driving. Gatsby says he had to put on the emergency break to stop the car.
We do not know what Nick is feeling towards Gatsby as the chapter ends. But by the next morning, it is clear Nick is back on Gatsby's side. We know this because he is telling him things again. In fact, he feels he must rush over to Gatsby's house:
I felt I had something to tell him, something to warn him about.
He advises Gatsby to go away, then lingers with him, missing two trains to work. When he finally must leave, he says he will call Gatsby at noon. Clearly, he has a premonition that something will go wrong.