Ehm, don't want to turn this into a philosophical debate but I need to say this... I don't think Gantz' purpose of explaining all of this was to turn them all into atheists, they'd be a few centuries too late for that. "Meaning of life doesn't require afterlife or even validation from a God"... a lot of the fighters would already think like that. Gantz is trying to emphasize a deeper kind of meaninglessness.
I'm still trying really hard to figure out WHY Gantz is explaining so much though, I honestly can't figure it out. First I had this theory that Gantz was trying to stomp out humanity's will to evolve so they wouldn't turn into a Type III civilisation that could rival all others, by saying "we've done that, it was pointless, don't hope for it". But Gantz could probably wipe out all of humanity if they wanted to. They don't fear humanity's evolution and if they did... they could easily stop it.
I then considered the old theory that Gantz is still playing games, ending a TV show with a big trollish mind**** of the maincast. But they seem too evolved and too cold for that kind of sadistic amusement.
In the end, this just looks like Oku is trying to write a story about how not-special we are, feels like he's repeated it over and over again that there's no difference between us and the animals that we eat or even the minerals we use for tools. Perhaps to make us more humble... that or he's just trying to make people feel trolled AND depressed.