Hard G. I’ve been in the tech field all of my adult life and the only time I ever heard “Jigga” was in BTTF.
Let’s try it on for size:
My laptop has 512 jigs of ram!
Hey lookie here at this 2 jig thumb drive!
Nope, doesn’t fit.
In German it would be pronounced with a harg G.
Googly.
Yep, So let us do it right: Gigga please…
… I’m terribly, terribly sorry.
Moderator’s Note: Since the “factual answer” has already been given (either pronunciation is acceptable) but people still seem to be discussing the issue, moving from Great Debates to IMHO.
Telecom tech here. We have many circuits at a Gig or more & I’ve only heard it pronounced with a hard G.
A Gig’s worth of ethernet is called a Gig-E. Imagine if it were a Jig-E.
Many years in the frequency control industry and I never heard anyone say “jiga.” It was always Giga, as in GigaHertz.
Can we argue about GIF v JIF now? I say like “gift” - t. What say y’all?
Giraffics Ingterchange Format?
“What do I get with this ethernet card?”
“Well, a 2-year warranty, and you get Jig-E with it, of course.”
I’m wit’ youse!
However, now I must know: Is your username pronounced Fugayasi or fujayzi? 
I always pronounced the band-name Foo-Gah-Zee.
:smack: :smack: :smack: :smack: :smack: :smack: :smack:
Studied physics in the 70’s and worked as a physicist in industry for the last 25 years. I was taught “jigga” and that it shares its root with “gigantic”. When hard drives got bigger than hundreds of megabytes, I heard the young salespeople at Best Buy say “giggabyte” as if it shares a root with “giggle”, which I did. But that’s the first I remember hearing it that way. My older dictionary has the soft g.
Like I said in the original thread, if your standard is the original Greek word, then γίγας is pronounced with a hard “G”.
I will immediately start using the retro, ‘Jiga’, version for the sake of being retro.