For people who are wondering, it comes from the book Going Postal, one of the Discworld series.
The book features 'the Clacks' a sort of primitive telegraph system run from high towers that Terry Pratchett descibed as "sort of what the internet would be, if all you had was a lot of bits of string" (I'm paraphrasing here from what I remember from the intro to the film). Messages are sent from one tower to another using flashing lights in morse code, until they reach a tower within walking distance of the people they are meant for.
The people who own the business are so set on providing The Best (and most expensive) Service, that they refuse to stop the machinery so as to allow time for maintenance or proper breaks for the workers, leading to multiple people falling to their deaths due to exhaustion/poorly maintained towers. The actual plot of the book is about an ex-conman trying to rebuild the historic city post-office, so as to destroy the Clacks company's monopoly on long distance communication.
Thing is, whenever a Clacks worker dies on the job, the Clacksmen (and women) have a tradition called 'sending home', where the person's name is sent through all the towers, ending at the one closest to their home. The exception is John Dearheart, the son of the man who invented the Clacks system originally (and then had it stolen from under his nose), whose death is rumoured to have been a murder at the hands of his employers.
His name is sent around along with the code 'GNU', which is a signal to the Clacksworkers recieving it that they are meant to always send it on to the next tower, whatever happens. The idea is that, since a person is said to only truly die the last time their name is spoken, if John Dearheart's name is constantly being 'spoken' by the Clacks Tower mechanisms, then he's not really dead.
After Sir Terry's death, fans picked up the idea. GNU Terry Pratchett essentially just means 'remember Terry Pratchett'. You're meant reply with the same, so as to ensure that the name never stops being spoken, and so he lives on.
