I was stuck there too.
Endgame / Good ending spoilers
Proceed to the end boss, but just damage him until the moon turns red. Then equip his sword and cut the moon with it.
Afterwards, go where you fought Zangetsu the first time (garden of silence) and cut the moon there.
You're really close
Read the description of the sword
more specific
Fight Gebel with the sword equipped. When the moon turns red attack the moon with the Zangetsuto.
Thank you!! I indeed should have read the description lol