I live for my cat, genuinely. He won't understand what happened, he'll think I just left him behind. He'll get grumpy if I leave for too long. I don't know at what point his agitated sulking would turn into confusion/grief, but imagining this is unbearable. His lifespan expectancy is shorter than mine, so I will endure any amount of suffering to be present for him. I'll probably end it right after he goes, imagining life without him seems unbearable. He's been my rock for 12 years
So I have empathy for you, and encourage you to stay for Gizmo. Even if everything else seems pointless, just make it your life goal to make his life the best it can be. Make his joy your joy. I don't know what your life is like, but if you've got a cat that makes you happy, it's hard to imagine it's truly entirely horrible. It's also hard to imagine a good owner can put their trust in others to make sure their pet is loved like they are now.