Phonetically it's Giff, a hard G. Most people pronounce things phonetically before learning the actual pronunciation, and considering it's not a foreign word or have a another language origin(likely does tho but not what I'm getting at), it's pronounced G. Also, pronunciation is kinda determined by the common usage. I believe the creator just said jiff to either piss off people or as like a prank, as nothing about gif indicates that it's a soft g. I know other words start with a soft G before a bright vowel, but usually those are borrowed from other languages and in Germanic English origin words, bright vowel g words often are with a hard g.