HELP?! Help me find this word?

Okay, uhhm Ima try my best to describe it. Please help me figure what it is. Okay, so this is a really old thing that they did along time ago. Like when people committed a crime, they would do punishments. They would put peoples head in a hole, and like drop a blade from the top, to decapitate the head, what is it?! I know it starts with a ''G'' i think!