I feel like Gatsby enjoys being the "Prince of darkness" but like when daisy dies of old age Desperately tries to either turn nick into a vampire or tries to convince him to or like tries not to bite him but would be extremely depressed because he loves him and doesn't wanna see him die
Oh my god totally, I HC him as having dependency issues and his greatest fear is being alone and just that concept terrifies him and being with Nick makes him so happy and those fears are kinda pushed to the back of his mind and he trie not to think about them. But once Nick and Daisy and Jordan all start to visibly age and get old (like grey hair) the reality of his situation, that being he’ll outlive everyone he cares about, becomes a lot more real. And definitely by the time Daisy dies of old age he tries to get Nick to become a vampire so they can be together forever. And Nick for some reason doesn't want to and Gatsby respects his choice but also at the same time is agonized over this and has moments where he’ll be lying awake in bed with Nick by his side sleeping just thinking about how easy it would be to just turn him right there. How maybe Nick wouldn't be that upset when he woke up and realized what Gatsby had done. Maybe it would be the best thing to do so they could live together forever and never have to deal with the heartbreak of losing the person they love.
But Gatsby never acts on these thoughts, instead pushing them away, remembering what it was like to be turned to a vampire without the choice and could never do that to Nick, who he would happily die for if need be. So he watches. Watches as Nick grows old, hair turns grey and recedes, his face becoming covered in wrinkles. He helps Nick get around and do things when Nick starts to have difficulty doing things he had been able to do before by himself. Watches and Nick dies peacefully in his sleep, take his last breath. Watches as he's buried in the ground, finally joining their friends in death.
Gatsby going back to their house, which seems so much more cold and empty now. Laying down in their bed but finding it too big and feeling like somethings missing.
And then decades later still visiting Nick’s grave every year and placing flowers on his grave. Wanting to make sure that Nick is never forgotten, that his grave never becomes covered in weeds, that his tombstone looks as clean and polished as it did on the day that Gatsby buried his soulmate