One of my favorite things in ff7 rebirth
Is that they fleshed out the Gi tribe.
In the OG game, the Gi were just random enemies encountered in Cosmo Canyon that had tried to invade at some point and were pushed back (and apparently wiped out), as you fight them in present day as ghosts. That was it, thats all the info we got.
In Rebirth, they've been significantly fleshed out.
A race older than the Cetra from a world that existed before The Planet. Gi Nattak explains:
"With the passing of eons, the star we called home began to wane, until at last it was subsumed by your own. The earth shook, seas boiled, skies shattered, and time stopped. Few of my people survived the chaos and calamity. Those who did began a new life here
But, to the Planet, we were not welcome. Not in life, nor in death."
We also learn that that which did not come from the lifestream cannot return to the lifestream.
The Gi were an alien race that lived on a world that came to an end, possibly because of the birth of Gaia. Gi refugees settled on Gaia, but not being of the planet, were unable to die and join the lifestream, and so now live eternal, tortured lives as spirits. To that end, they laid claim to a particularly powerful and pure materia, and prayed upon it in the hopes it would grant their wish for "freedom". And their prayers eventually worked. After milenia of praying, collectively pouring their desire into this purest of materia, the materia "began to stir".
Chanting heard throughout the Gi's village tells us everything we need to know about this materia of theirs.
"Steeped in our one desire, purest of materia no more, with pain and spite made black"
The Cetra then somehow learned of the black materia and stole it away from the Gi, hiding it away beyond their reach.
At no point does Nettak tell the party "Yo this materia will destroy the world lol" he intentionally does not tell them what exactly it does. Just that its very powerful. The Gi refer to it as "The materia of peace" and the materia that will grant their wish for freedom. The Gi want Cloud and the party to retrieve their black materia for them, so then why would they tell them what it does?
Now, my personal headcanon/theory is that maybe the Cetra learned of the black materia because the Gi actually used it. Maybe, roughly 2000 years prior to the game's events, they called down a meteor that, along with scarring the planet (the northern crater) brought with it something unexpected
