Moune said:
If it'll end the next chapter, how will they end it?
Poorly.
Really I can see it going one of two ways, with the actual outcome most likely falling somewhere in between:
Happy Ending: They land a decisive blow on the alien and whoever is remaining from that fight gets off the ship and lives happily ever after, perhaps using Gantz technology to form a Earth Self-Defence Force to protect the planet from future alien attacks.
Bad Ending: The remaining survivors all end up dying aboard the spaceship after taking down the final alien threat, leaving Tae to cry over "KEI-KUN!!!" for the rest of her existence, Ayumu growing up without his brother and the Osaka lady surviving without her teenaged heartthrob. The rest of the world is left in ruin but will slowly rebuild itself, as that's what humanity does after a disaster.
In any eventuality I have a feeling this will be a hot contender for worst manga ending I have ever read (which includes the cop-out of an ending that Tenjou Tenge had).