It's important in first person novels to question the point of view of the narrator, especially their self-assessment, for who among us can see ourselves clearly? As readers, we question the point of view or the reliability of a narrator by pitting what happens against the narrator's assessment of what happens.
In Gatsby, Nick's assessment of himself as an honest person doesn't hold up under scrutiny—we know, from his own admission, not to mention an early dinner conversation with Tom and Daisy, that Nick is not being entirely honest with the "girl" from Chicago to whom he keeps signing letters "love" while he is seeing Jordan—and we know as well he is remembering the Chicago girlfriend primarily in terms of the unattractive sweat mustache that forms on her upper lip after a game of tennis.
Nick has never felt too deeply for Jordan, either. He describes himself at the end of chapter three as "flattered" to be seen with her, and as feeling a "tender curiosity" about her. He thinks "for a moment" that he is in love with her, while also finding her shrewd and dishonest. He finds fleeting relief from the loneliness of turning 30 in her company. When tragedy strikes and Myrtle dies, Nick lacks the emotional commitment to her to give Jordan the support she needs. She reaches out to him, wanting him to stay with her, Tom and Daisy. She asks, "won't you come in, Nick," and he refuses. She calls him at work the next day, wanting to work out their differences and even offering to come "to town" from Southampton to see him, but he again refuses, saying he couldn't sit across a tea table from her "if I never talked to her again in this world." Instead, he tries to call Gatsby four times.
From these events, we can surmise that Nick is more interested in Gatsby than Jordan, supporting a queer reading of the novel, and we can see that Nick's feelings towards Jordan have been fairly superficial. He's never been deeply invested in her, no matter what he might want us to believe, yet it takes a tragedy to make this apparent. Jordan, imperfect as she might be, realizes this when she tells him he wasn't very nice to her the night he leaves her at Tom and Daisy's. When he refuses to see her the next day, that ends the relationship. There simply isn't enough there for Nick not to lump Jordan with Tom and Daisy as people he wants to avoid. Gatsby, in contrast, as he tells us throughout the novel, is the person who captures his imagination.