Actually, I'm kind of surprised by how this ship developed on its own. It wasn't something I ever thought about while reading the book, but fandom has a way of pulling out these background dynamics and just running with them. The most common trope I've seen is probably fix-it AUs where Gatsby survives, and Nick helps him escape the whole West Egg mess, maybe even going abroad with him. That one's always a bittersweet comfort read.
Then you get a lot of pining, obviously, because that's the core of Gatsby's character anyway. So many stories are just Nick watching from a distance, wrestling with his own feelings while Gatsby remains laser-focused on Daisy. It turns the novel's observational quality into this slow, agonizing crush. I've also noticed a weird number of ghost stories or supernatural twists, maybe because the original text already feels so haunted. Gatsby as a literal ghost visiting Nick years later seems to be a whole subgenre.
Honestly, a lot of it hinges on reading between the lines of Nick's narration—was he just fascinated, or was there something more? Fandom tends to believe the latter, and the tropes build from there.