This occurs in chapter 3. This is very important because Nick paints himself as the moral character of the story, and he should, he's the story-teller and has the right and ability to taint our viewpoints.
In the end of chapter 3, Nick notes Jordan's dishonesty. The he points out her carelessness in reference to driving. Finally, he recognizes something in himself, he is intrigued by her. BUT, HE HAD A GIRL AT HOME. Yes, there is only one line that really tells us this, but he kept it from Jordan until he thought a relationship was possible.
He reports to the reading audience:
"Her gray, sun-strained eyes stared straight ahead, but she had deliberately shifted our relations, and for a moment I thought I loved her. But I am slow-thinking and full of interior rules that act as brakes on my desires, and I knew that first I had to get myself definitely out of that tangle back home. I'd been writing letters once a week and signing them: 'Love Nick.'"
Later, in chapter 7, after the incident with the accident occurs, Tom drives into his home and Nick and Jordan are left out front. Jordan wants Nick to do something with her.
"I was feeling a little sick and I wanted to be alone. But Jordan lingered for a moment more... I'd be damned if I'd go in; I'd had enough of all of them for one day, and suddenly that included Jordan too."
Nick fails to tell her that he wants to be alone or that he is tired of her. But he tells the reader.
In chapter 9, Nick pretends to be surprised when Jordan reports she is engaged. He acts like he is happy for her, but he is still in love with her.