“Choosy moms choose Jif” is the slogan for a popular peanut butter in America.
When Steve Wilhite created the gif file format in 1987 at Compuserve, he would say “Choosy developers choose Gif”, a reference to the peanut butter slogan.
Gif stands for “Graphic Interchange Format” so its not surprising that it was pronounced by almost everybody else as having a hard G (in the same way we would say ‘gift’). However it should probably be pronounced with a soft ‘g’ as in ‘giraffe’.
If you do pronounce it with a hard ‘G’, don’t worry. The Oxford English Dictionary made “Gif” its Word of the Year in 2012 and in response to the fact we may all be saying it wrong, they have said that both pronunciations are acceptable regardless of what its creator, Steve, would prefer since he has little control over how people have gone on to pronounce it.
Or you could just spell out the letters … which is what the the National Spelling Bee’s official pronouncer does in the video in this link http://ftw.usatoday.com/2015/05/heres-how-the-national-spelling-bees-official-pronouncer-says-gif
While we’re at it, the developers of the PNG (or “Portable Network Graphics”) image file format have said it should always be pronounced “ping” and not “pee en gee”.