Japanese have some pretty wonky weirdness in their stories, fantasy, and history. Especially since they love to mish/mash everyone else's fantasy stories with their own. On top of that, they have some strange sexual concepts, oh which they aren't nearly as taboo about over there, as compared to many other places.
I like many "stories" and concepts they create, but the fact that EVERY japanese story has to have the following is a major annoyance.
This list is in no particular order:
Super shy, can't make up their mind main protagonist. (like the below archetype, this is the other one that is in every japanese story as most japanese people identify themselves as this archetype)
Festivals
Strange references to japanese ghosts or demons (also called oni) even in stories where it makes no sense to add them.
Watermelons + beach + summer + bikinis
Open air hot springs bath.
Tons of random food. Mostly japanese based, but most stories have several whole chapters dedicated to food and eating gatherings.
"Nya" female characters. These aren't usually any sort of main or side character, but something that just shows up and disappears quickly.
An over the top super violent character who tends to misunderstand all situations, but in reality is really good hearted person. (otherwise known to the japanese as tsundere archetype. There are several other major archetype, but this one is literally in EVERY japanese story where the others aren't.)
Potential harem or reverse harem
Panties
A "fall" that also makes two characters of the opposite sex get entangled in sexual situation that could never ever physically or conceivably occur.
There are many others that show up in the majority of stories, but the ones I put above show up in ALL japanese stories. They include filler areas in even the most "serious" of stories that will inevitably have all those above I listed be included in the story. Gantz is no exception the the above.
Gantz is also a pushing the envelope type story where more things are over the top in terms of violence and sexual references.