I may sound haughty and knowledgable when I say JiF then, but between just you and me, I didn't know a damn thing about this and just decided to say it this way in my brain for reasons that remain unclear.
Oooh, that's how grammar works in English language? Okay, so me as a developer of some obscure thing from this point forward are instructing everyone to pronounce "home" as "hume", since that's how you pronounce "o" in "tomb". I decided that solely because my software is loosely related to the meaning of the word. K thx bai.
Neither did the author of graphic format. GIF is not a word, but initialism, like NSA, FBI, NASA, IBM, etc. And there are specific rules how they are read and pronounced.
NASA is an acronym, not an initialism. And guess how the last letter of NASA is pronounced versus how the A in the corresponding word is pronounced. Ah vs Uh.
Irrelevant comment to the discussion at hand as the matter is not set in stone in English language. More to the point it doesn't change the fact how GIF is pronounced. Even if you consider it an acronym it's still a form of abbreviation and not a word on its own with known heritage, hence the general rules on how to pronounce letter g do not apply.
Had the word originate from French part of the English dictionary (like gin, giraffe, etc.) then g is pronounced as j before vowels e and i and would make sense. But Germanic words (such as gift, geese) still use hard g. So applying normal rules is pointless, since English has no such thing.
In short, it's pronounced whatever the way people pronounce it. End of story.
I always think all the arguments are ridiculous because it’s essentially saying that someone is pronouncing a product (not a word) that they created incorrectly. This product even has a catchphrase for it. There’s literally nothing you can say to contradict that. It’s a product with a catchphrase that describes how to pronounce it. If you pronounce it differently then you do you, but you are wrong.
It's also pretty funny when it's about actual product you will get corrected to the intended pronunciation, or at least, allowed because people acknowledge there might be multiple way of reading a word based on where you from. Like potato and tomato.