A separate dimension made up of various levels of liminal spaces (with threats of various kinds inside them). Kinda overlaps with the normal dimension and it’s very hard to get out of. There’s one story based on them on royalroad but it seems very comedic
I know this is off-topic, but I advise you to read it. It has it's problems, but I think upsides outweight them by a large margin.
As for the actual topic, I like Gantz a lot. So if you will offer me a similar story, I will read it. Not sure about other though. By other I mean SH readers, not SH forum users.