As was mentioned in the previous post, Nick Carraway is describing Gatsby's "unfamiliar yet recognizable look" as Tom Buchanan is telling Daisy that Gatsby is a bootlegger. The look that Nick Carraway is referring to is Gatsby's startled expression. Carraway goes on to mention that Gatsby looked as if he had just killed a man. Gatsby's facial expression is a combination of guilt and panic. As Tom gradually sheds light on Gatsby's illegal business ventures, Gatsby begins to panic because he realizes that Daisy will not approve of the way he earns his money. Gatsby's extravagant lifestyle is essentially a lie because he has paid for everything using "dirty money." Daisy is also under the impression that Gatsby's financial success came about legally. As Tom begins to expose Gatsby for the fraud that he is, Daisy slowly distances herself from him. Gatsby cannot bear to listen as Tom exposes him, and a panicked, guilt-ridden look appears on his face.
That look appears on Gatsby's face whenever he is faced with a truth that he would rather avoid. The first time is when Tom mentions "drug stores". This is a reference to the illegal alcohol sales Gatsby is involved in at various drug stores. The second time is when Tom indicates that a man named Walter has some information on Gatsby betting illegally. The look would best be described as the "hand in the cookie jar" look. These are moments when Gatsby knows that he has been caught out, and realizes he might have trouble holding the high ground in his argument with Tom.
I would say that it is the look of bewilderment previously seen at the end of Chapter 5.