What is Gatsby's response to Nick's compliment?
I assume the compliment to which you refer is the one in chapter 8 when Nick tells Gatsby, as Nick is leaving Gatsby's house: 'They're a rotten crowd. You're worth the whole damn bunch put together." There is no reaction mentioned. We know, however, that Gatsby valued Nick's opinion of him and that Gatsby wanted Nick to think well of him. We know that because of all that Gatsby told Nick about his life. There are probably very few people who knew all about Jay's background. Based on the gossip and wild rumors about him that we hear from others earlier in the story, we know that most people didn't know him much at all. Nick, however, has been told by Jay, about Jay's life. Also, Jay asked Nick, in chapter 4, what Nick's opinion was of him. That tells us again that Jay valued Nick's opinion, and not just because he was Daisy's cousin. Nick is level-headed and as he tells us at the start of the story, one who tends to reserve judgement. Based on all of that, Jay was probably very happy to hear Nick's compliment to him. It is fitting that those were the last words Nick ever said to Jay Gatsby.
The morning after Myrtle's death and after Gatsby has stood vigil all night at the Buchanans' in East Egg, ironically to protect Daisy from whatever might befall her, Nick goes over to Gatsby's house. They have breakfast; they talk. When Nick leaves, he turns impulsively and calls out to Gatsby:
They're a rotten crowd . . . You're worth the whole damn bunch put together.
Nick remembers Gatsby's response in detail. This would be the last time he would see Gatsby alive:
First he nodded politely, and then his face broke into that radiant and understanding smile, as if we'd been in ecstatic cahoots on that fact all the time.
Nick says he had "always been glad" he had said that; it was the only compliment he had ever given Gatsby, it had made him smile, and it had been the truth.