Answer: They are ghosts.
The people of Glubbdubdrib are a race of magicians and sorcerers. Among their power is that of raising people from the dead. However, they can only do so for 24 hours, and they can only bring back a specific person once every three months. These characters come back as ghosts, and during Gulliver's stay, he meets characters such as Brutus, Julius Caesar, Hannibal, Socrates, Aristotle and Homer.