The Room of Truth is a large empty room with 2 giant naked humanoid bodies with shape shifting faces. It answers any questions. Upon arriving, everyone has their eyes bleed for a time before they get used to it.
When asked about Gantz and all of it, they explained they were from a world which was targetted by the hostile aliens. After repelling them, the hostile aliens decided to change their target to Earth. They then sent military data to Earth to help them fight against the hostile aliens. They had nothing to do with the games, the points, or anything, they only gave the technology to build things and information about their enemy.
When asked about souls, they were told the soul is just data, stored somewhere when they died, and would be reincarnated as someone else later on. To prove this, Kei Kishimoto, Sei Sakuraoka, and Reika Shimohira were brought back to life briefly, then killed again.
The aliens then refused to answer any further questions, teleporting all of them back to where they came from.