The key to understanding the quote specified in the question -- "She was the first 'nice' girl he had ever known" -- lies in one's memory of having read the chapters that preceded the one (Chapter 8) in which this quote is found. Specifically, one should reread Chapter 6 of F. Scott Fitzgerald's classic The Great Gatsby. It is in that chapter, that the author, through his narrator, Nick Carraway, provides the essential background on the character of Jay Gatsby. It is in Chapter 6, that we learn that this mysterious figure was born "James Gatz of North Dakota," and that one of the transformative experiences of his life was his encounter with the yacht owned by Dan Cody. It is the encounter with Cody that leads Gatz to adopt an entirely new persona, that of Jay Gatsby, and to enter the only world he knows that can provide the financial resources necessary to make his new life, under an assumed identity, work out. Fitzgerald's biographical information on James Gatz includes the following passage that provides the most important clue to the meaning of the "'nice' girl" reference:
"He knew women early and since they spoiled him he became contemptuous of them, of young virgins because they were ignorant, of the others because they were hysterical about things which in his overwhelming self-absorption he took for granted."
In short, Gatz/Gatsby is presumed to have known and slept with many women, none of whom he held in particularly high esteem. And it is this history of sexual conquest without romance that provides the context for the paragraph in Chapter 8 that includes the following passage:
"She was the first ‘nice’ girl he had ever known. In various unrevealed capacities he had come in contact with such people but always with indiscernible barbed wire between. He found her excitingly desirable."
And so begins Gatz/Gatsby's obsession with Daisy. The Great Gatsby is about a number of topics and themes, but underlying it all is Gatsby's infatuation with and quest for Daisy Buchanan. James Gatz's first true love was the Daisy of his younger years, the one who would marry the wealthy Tom Buchanan and, consequently, be elevated out of James' reach. The irony, if you will, of Gatsby's use of the word "nice" in this context is that, as the earlier passage about the conquest of virgins suggests, what he loves most about Daisy is that she isn't the personification of virginal purity; in fact, she is the opposite:
"It excited him too that many men had already loved Daisy—it increased her value in his eyes."
Whether the quote regarding a "nice girl" reflects Gatsby's romanticism, cynicism, or materialism is open to interpretation. Gatsby's life has been driven by his materialism, evident in his relationship to Cody, but he is also quite the romantic. When Nick first spies his new neighbor, the latter is poised on his lawn reaching out for something beyond his grasp. As we know from that green light at the end of the Buchanan's dock, we understand that James Gatz/Jay Gatsby is very much a romantic at heart.