I'm trying to ID an old science fiction story featuring an (older) resistance fighter whose grandchildren tagged him something like Grimper or Gramper.
He left Kilroy-style graffiti with that name, which terrified the alien invaders.
The novel is Sleeping Planet by William R. Burkett, Jr.
Alien invaders spray Earth with chemical that puts everybody to sleep, with a couple of exceptions. One captured awake human starts spooking the alien troops by telling horror stories about the vengeful ghost of his dead grandfather. The aliens mispronounced the name as "gremper".
Meanwhile on the ground a second awake human gets wind of this and starts a psyops campaign pretending to be the dreaded gremper.