The premise is different, but high school of the dead is another Post-apocalyptic manga where humanity is on the brink. a few chapters bear a lot of similarity to how sheltered people treat those that have survived in the now violent world.
Although at first Gantz starts off at a small scale, the later chapters become similar to the setting in ayanashi in the idea of humanity being cornered. Its a long read, if you dont enjoy gantz itself, then it wont be worth reading all the way up to the point that led me to make this recommendation. Otherwise, please enjoy.