According to the words spoken by Kouzou Fuyutsuki in episode 26', the opening of the Chamber of Gaf is synonymous with the opening of the door to the world's beginning and end. When this door opened, the surface of the Earth turned to LCL, and people were also no longer able to maintain their individual life forms and were reduced to LCL. Ritsuko Akagi also spoke of the Chamber of Gaf in Evangelion episode 23.

Source: The End of Evangelion : Glossary contained within The End of Evangelion - Theatrical Program.