A first few conflicts with Gatsby and Nick occur during chapters 3-4. Nick is learning about Gatbsy's character and does so quite a bit through rumors. As chapter 4 comes around, Gatsby tries to clear up all the rumors on a road trip to New York. Nick actually grows quite annoyed with all the insinuation about a certain "matter" he wanted Nick to help him attend to. Once they get to where they were going, Nick meets a work partner of Gatsby's and feels like he is led to be involved in some dirty deals, but wants no part of it. Nick is losing faith in Gatsby.
In chapter 5, when Gatsby arranged to meet with Daisy at Nick's house, Nick is very welcoming, but notices that Gatsby is extremely nervous. So, when the two men are in the kitchen, Nick refuses to go back out into the room where Daisy is and he just has to deal with it.
In chapter 8, Nick's conflict with Gatsby has to do with wanting Gatsby to understand the reality of his new state of relationship with Daisy. Gatsby thinks everything will just blow over in the morning and everything between he and Daisy should be fine. But the fact of the matter is that he and Daisy just committed murder together. Nick struggles within himself as to how to tell this to Gatsby. Nick comes to the realization that Gatsby just won't get it, so instead he recommends that Gatsby get out of town for awhile. This is about 2-3 pages into the text of chapter 8 depending on your book.