Nick's comment on "fundamental decencies" shows that he understood that his father's advice was within society's moral values, however, he allows himself to circumvent that advice and issue judgments, but keep it to himself.
We can arrive at this answer because:
So Nick can see non-judgment as part of the "fundamental decencies" established in society, which are presented as correct, but which are not always followed by everyone.
Nick is the narrator of "The Great Gatsby" and you can find more information about it here:
https://brainly.com/question/14334031