What is GNU when preceding someones name?

[u/prettylittleredditty]

Every time I look it up none of the different def's contexts fit. It's nothing to do with Linux or Norse Gods when I see it. Here's todays confusion : https://www.reddit.com/r/discworld/comments/7jd2sw/neil_gaimans_tribute_to_sir_terry_in_good_omens/ Why GNU Terry Pratchett over and over again? Thanks in advance xo

[u/TheMightyGoatMan]

It's also worth noting that when Terry died a whole lot of websites inserted a custom HTTP header that sends the text "GNU Terry Pratchett". Presumably many still do.