[npteljes]

One of Wiki's sources date it back as far as 1994, and that's a news article, so the thing must have been going on for a while.

Thinking about it, I think I understand why hard G makes sense for people. With GPU, we pronounce the the individual letters, as it's clearly an abbreviation - as no sane English word starts with "gp". With GIF though, even though it's an abbreviation, it looks a lot like a normal word, "gift", and English also has "give", another one with a hard G, so it feels familiar to say. Moreover, the US, where GIF comes from, has Jif already established as a peanut butter brand, so it makes sense to not pronounce a newly invented, differently written word the same as an already established thing. Well, at least to some it makes sense!