Gantz is a difficult manga to adapt, primarily because Oku Hiroya makes up the story as he goes along. Typically the adaptation does its own thing after the Buddha mission, with an underwhelming result – which reveals my opinion of the second movie.
Gantz Perfect Answer proceeds strongly for much of its length – the encounter with the “black-clothed aliens” is fast-paced and excellently choreographed (the swordplay is distinctly unrelenting and aggressive in style) – though they aren’t identified as vampires nor is an individual close to Kurono Kei involved. The survival of one of the aliens is a great surprise too.
It’s the ending where Perfect Answer missteps: the movie can’t figure out the morality of the missions, provides a poor justification for the final mission, and moves into a mediocre finish. It’s unpolished and confusing. The movie would benefited from keeping the final battle clean (leaving the major kills to major characters) and the antagonism clear.