She was the first “nice” girl he had ever known.
“Nice” is this sentence refers to Daisy’s wealth, meaning Daisy is the first “Old Rich” girl that Gatsby had ever met. This is one of the reasons, if not the reason, that Gatsby falls so hard for her.
Nick, who places a great deal of value on traditional “cardinal virtues,” is drawing attention to society’s casual conflation of good character traits (“nice”) with social status.
To help improve the meaning of these lyrics, visit "The Great Gatsby (Chapter VIII)" by F. Scott Fitzgerald and leave a comment on the lyrics box