Something that's gone isn't here anymore. If the sun is gone from the sky, it has set for the night and will rise again the next morning.
Things and people that have departed are gone, and times that have passed by are gone. When there's nothing left, it's also gone: "Sorry you got here so late — all the pizza's gone!" You can also use the adjective gone to mean "died" or "passed away," as when your friend tearfully tells you that her beloved goldfish is gone. Gone is from the verb go, from the Old English gan, "to depart or go away."