I think when Nick says this in the introduction it explains their differences, and the way that Nick sees Gatsby.
“If personality is an unbroken series of successful gestures, then there was something gorgeous about him, some heightened sensitivity to the promises of life,…[Gatsby had] …an extraordinary gift for hope, a romantic readiness such as I have never found in any other person and which it is not likely I shall ever find again.”
THere is also this exchange which highlights their differences:
“You can’t repeat the past.”
“Can’t repeat the past?” he cried incredulously. “Why of course you can!”
Nick is presented as the loyal friend, observer, peacekeeper in the book, while Gatsby is the always optimistic, ever hopeful character who wants to "fix" the past. Nick is not cynical, but realistic about what can and cannot be changed in life, while Gatsby thinks if he can just go back to the way it all was before, everything will work out just fine. Nick gets drawn into the controversy of the novel by becoming a "partner in crime" with Daisy and Gatsby, as is shown in the party scene of Chapter 6. At Gatsby's last party of the summer, Daisy tells Nick that she and Gatsby are going to "take a walk to the bungalow" and to "watch out for fire, flood or act of God." Later in the chapter, Nick dissuades Tom from going over to the bungalow, protecting Daisy and Gatsby's activities.