Fun question. Nick has complex feelings towards Gatsby. His own opinions of him change, which is part of what makes him an unreliable narrator. The more he understands about Gatsby’s past, his illusion of refinement, and his love for Daisy, the more Nick seems to respect him and admire him as a romantic hero of sorts. By the time of Gatsby’s death, Nick’s admiration for him is in full force. If he were to eulogize Gatsby, I think he would do so very respectfully, perhaps even wistfully.
More importantly, he would probably not reveal any of the specious characteristics he knows about Gatsby. He would likely not mention Gatsby’s love for Daisy—largely out of respect for her and also for him. He would probably say that Gatsby had a very tragic death, which is difficult to make any sense of. Then, knowing Nick, he might go off on a bit of a tangent about how little sense anything makes in the world, nowadays.
One characteristic about Nick Carraway is that he is not particularly sentimental. It is difficult to imagine him tearing up or lashing out at a Gatsby funeral. He would probably be contained, smooth, and eloquent, just like the man himself.