Disappointing, great ideas, but the execution was horrible.
With the Gantz technology they could just revive ANYBODY, but conveniently it doesn't work like that anymore. Why? Because the mangaka said so, plausible.
If an author creates rules within his world, he also has to play by them and not ignore them, when it suits him. You can't make up a rulebook and than disregard it the second you don't like it anymore. You should have put more thought into it in the first place then or at least try to fix it afterwards.
This is why mangas in Japan have editors, those are the guys who have to step in and tell the mangaka that this isn't working, at all, and they have to redo or overthink the way the story is suppose to unfold, otherwise it will be a trainwreck, which it has become. Apparently Oku Hiroya's editor was afk for the last years while publishing it. It seems like Oku Hiroya didn't had any real idea where he was going with this manga and wrote it just the way it came to him, thus you have bullshit like vampires or some blonde giant, who kills off his entire species, just because Kuruno killed his brother, absolutely reasonable.
If this entire alien species, those Giant Aliens, were really desperate to migrate to another planet, why the fuck didn't they try to communicate with humanity first? That Fraa Alienwoman was able to use a Google Translator within minutes and realized after barely one hour that their species aren't that much different. What was there to gain by fighting, killing and eating them in the first place? and in the end they are all dead, because of some stupid honour.
Yes, we humans have done similiar shit in the past (and in some parts of the world we are still doing them), but not even we are that stupid to kill our entire species for the sake of honour or whatever. Our own survival is more important than anything else and that was my impression of those aliens as well, but in the end they still suicided and killed themselves anyway (and probably a lot more like other animals and so on on the ship). It doesn't make any sense whatsoever.
Really disappointing how logic is ditched for some weird cool-looking edgy gore pictures. It's like a 15yo wrote a story without giving any thought about how it makes sense in any way.