Explain this quote that Nick says to Gatsby. What does he mean by this? "'They're a rotten crowd,' I shouted across the lawn. 'You're worth the whole damn bunch put together'"

It is clear throughout the novel that Nick comes to view Gatsby as a man with noble character. Nick gets sucked into and enthralled by the world that Gatsby lives in, and also comes to understand his fascination and obsession with that world; Gatsby has come by his fame and fortune by "humble beginnings" in Nick's eyes. He wasn't born into a wealthy family, but was a fortunate recipient and thankful patron of the life he lived.
Nick also realizes throughout the novel that none of Gatsby's aura of golden living was for himself, but for love. Gatsby is humble, generous, welcoming, and vulnerable to Nick. Although he is a part of this "rotten" social circle, Gatsby is vastly different than Tom and Daisy and their own obsessions of wealth, appearance, and shallow friendships. Gatsby can never truly fit in with the crowd of old money, taught class and arrogance, and selfishness. He was born into a poor family, an army man, and had to work his way into the whirlwind of a society he now belongs.
Understanding all of this, we can see why Nick views Gatsby as "worth" more than "the whole bunch"; Gatsby has morals, character, and a conscience compared to his constituents. Nick sees his good heart and his vulnerable soul as opposed to the hollow and counterfeit individuals he surrounds himself with.

This remark shows that Nick has been entranced by the "world of Gatsby". This scene occurs in Chapter 8. Nick and Gatsby had just had a long talk about Gatsby's past history. Gatsby did not feel worthy of Daisy because she was of a high social status, and he was not. Nick hates this. He hates this because he does not feel fondness toward the ultra-rich, famous, but empty people who populate Gatsby's parties.
He comes to the conclusion that Gatsby is worth all of the so-called "friends" who flit around him, leeching off of his money, drinks, music, food, and excesses. This is said the last time before he is killed. By this point, Nick's character has developed. He has already made his judgments about the other varying characters in the book. He has particularly made up his mind about the upper-class people, Daisy and Tom. Since the beginning of the story, Nick has become more vocal about his opinions.
Before this quote, we were aware at just how much Nick treasured his relationship with Gatsby, but not how Gatsby feels about Nick. In this instance, he smilingly shows Nick just how much his relationship with Nick means to him.
He has finally realized what "true worth" really is. To Nick, true worth isn't the money, the big homes, the parties, the cars, the food and drink...but the individual's soul and spirit determine his or her true worth.

Among other things, the statement shows just how much Nick has been sucked into Gatsby's world, like so many others before him. Previously, there'd always been an air of detachment about Nick, a sense that he was somehow above the fray of Gatsby's strange, exciting world. Yet even he has succumbed to its charms, drawn in by a combination of Jay's charisma and the fundamental decency of his character.
By complete contrast, Nick utterly rejects the world of Tom, Daisy, and Jordan and the shallow, vacuous phoniness it represents. He respects Gatsby all the more because he's not really one of them. Gatsby is an outsider: someone who'll never be accepted by the old money elite of East Egg. And for Nick, that's something to admire. Although Gatsby may mix socially with such people, and although he clearly aspires to be accepted by them, he still retains some integrity.
The key word in Nick's statement is "worth." Gatsby may not have been born with a silver spoon in his mouth; he may have derived his phenomenal wealth from the proceeds of organized crime; yet for all that he's still worth a whole lot more than all the Toms and Daisys of this world put together. Nick has changed because, thanks to his relationship with Gatsby and his negative experience of running with the East Egg crowd, he's finally realized where the true worth of someone really lies.