Valid question, and not easily duckduckgoable. I found this explanation on tumblr, by blackboardmonitor. I wish I knew how to link to the damn thing, but I can't figure out the tumblr interface.

    In Terry Pratchett’s Going Postal, the operators of the clacks
    tower use “GNU” as a code to mean the message should be passed
    onto every tower. An article on wiki-Lspace says that “The G in
    the code means to send it on the U at the end means to turn the
    message around at the end of the line and the N means not logged.”

    When a clacks operator died while working, or was killed, their
    name was passed in the overhead with “GNU” in front of it, as a
    way of commemorating them, of not letting them die, because, “a
    man is not dead while his name is still spoken”. It’s a way of
    keeping them alive, you see.

    Terry Pratchett, may he rest in peace, wrote the clacks as a
    fantasy equivalent of social media, of emails, in particular, but
    general interweb communication as a whole. Therefore, people are
    posting “GNU TERRY PRATCHETT” because it’s the equivalent in our
    world of commemorating him on the clacks; we’re keeping him alive,
    in a way, in a way which I think he would have liked.

    Because no one is really dead until the ripples they caused die
    away. So, now & until I myself die: GNU Terry Pratchett.