I was, a few years ago, priviledged enough to have a very brief run-in with one of the co-creators for the GIF format, Terry Welch -- the W in LZW. My uncle was working for Unisys, and I was there for some reason or another and in the process of showing me around the place we ran into the guy.
We didn't have much of an intercourse, but in the few seconds I had with him I asked him the question about pronouncing 'GIF'. He told me that while he pronounces it with the stressed 'G' ("g-if"), the other two people (the "L" and "Z") use the 'J' convention, ("jif").
Readers are invited to make their own judgement.