Ambiguity means unclarity. Nick opens this story preparing the reader for a flashback.
His first words about Gatsby are these:
Only Gatsby, the man who gives his name to this book, was exempt from my reaction - Gatsby, who represented everything for which I have an unaffected scorn.
Just prior to these words, Nick told the audience about this unaffected scorn. He was mad at what he had experienced in humanity. He comments that he longed for a "sort of moral attention forever" and that he did not want to adventure any further into the dramatic unfoldings of the depths of human hearts. Obviously Nick had some bad experiences which made him question the morality of humanity. He had unaffected scorn for this attitude and lifestyle. To me, that says that he is repulsed by the immoral.
What seems strange here is that he holds Gatsby as exempt, meaning Gatsby did not receive Nick judgment or scorn. However, in the same sentence he says that Gatsby represented all of it. All of the immorality and dramatic toil in the human heart was embodied greatly in this man, Gatsby. So the ambiguity comes with why Nick would not hold Gatsby with great scorn if Gatsby did everything that Nick hated.