This probably won't be a fair nor deep review, but it's the kind of review I needed when I was considering reading this manga. Gamurakan is... quite the horror manga. The synopsis really doesn't tell you a thing about the story but there's no synopsis that could ever do so. If I try my best to express my experience reading Gamurakan without spoiling anything, I'd have to say it was... Boring. I found it boring. I can trace scenes and images that would be very interesting for people who are into lovecraftian horror or brainstorming solutions to unanswered questions, but personally it just wasn't my cup oftea. What makes me have fun with horror is how interesting the plot is and the tension the story can build. Gamurakan doesn't have that, which doesn't make it straight up bad, but simply not everyone's style.
Gamurakan is a strange bowl of body snatchers mixed with cosmis horror maybe? A lot of sexual content and just a very (very) few Junji Ito-ian styled images. You won't really know what happened, how is Gamurakan's world built, nor probably remember any characters. If you're in for weird horror, go on. If you were expecting an interesting plot with creepy looking girls, this isn't it, chief.