Why does Nick say, “They’re a rotten crowd… you’re worth the whole damn bunch put together”? Why is good that he chose this moment to say this to gatsby?

This happens in Chapter 8. Gatsby and Nick have a long talk about Gatsby's past. Gatsby confides that initially he just wanted a bit of action with Daisy but fell in love. From then on he never felt worthy of her because of her high social status. This sickens Nick because he hates these vacuous ultra-rich people. He thinks Gatsby is worth more than all of them together (hence the quote). Nick laments that Gatsby is pining away for the shallow Daisy when he could be making more money and throwing lavish parties. Nick loved the old Gatsby from the beginning of the novel. He liked the mysterious paradoxical counter-culture that Gatsby represented. I like this because everything about East Egg is about money yet Nick is talking about a different kind of worth; he is saying that Gatsby is worth more than money compared to these superficial people.

I am not sure why my answer is getting deleted but the above response is not entirely correct. Nick doesn't "love the old Gatsby from the beginning of the book." In fact, even in this direct quote, he emphasizes how much he has always disapproved of Gatsby: "I’ve always been glad I said that. It was the only compliment I ever gave him, because I disapproved of him from beginning to end."

This moment was significant for numerous reasons. Firstly, in the beginning of the novel, Nick says that he is "inclined to reserve all judgements." However, as the book progresses, it becomes evident that, while Nick does not necessarily approve of Gatsby, he understands the wickedness and corruption regarding East Egg ("they're" is referring to East Egg - Tom, Daisy, etc.). East Egg represents old money - privileged, superficial, corrupted, materialistic and not welcoming to anyone outside their circle. This is the first time that Nick makes a clear judgement of his opinion. He acknowledges that Gatsby is morally "worth" more than the old money type that Gatsby ironically aspires to be.

The quote is also significant because it is the first real moment of friendship that Nick displayed to Gatsby. It was a heartfelt moment that took a break from the drama regarding Gatsby and Daisy. Although Gatsby did not know he was about to be killed, it really symbolized that Nick was really the only one was not that was not completely corrupted and truly understood him until the very end.