u/Expose_Them_

There seems to be a bug in the lastest Version.

In the Shinra manor when you break the box with the gravity Materia, Obtain it, then go up the floors and fight the Yin & Yang boss Solo as cait sith. Equip the gravity materia in your slots

Apparently if you die and have to restart the battle once, It will wipe you ever obtaining the Gravity Materia in the first place after you complete the boss battle and return back to Cloud and party.

Resulting in the Gravity Materia NOT appearing in inventory thereafter.