When I got my first PC and came into the world of hard drives and operating systems, I used to pronounce Giga with a soft G, or J sound (as in giant). Don’t ask me where I picked up this habit (maybe from the Doc in Back to the Future or maybe from Singapore), but that was the accepted sound for the unit used to represent a billion, as far as I was concerned.
Hence, it was a cause of much embarrassment when people made fun of me for this “wrong” way of saying gigabyte or gigabit and I soon switched to saying Giga with a hard G (as in giggle).
It seems that I didn’t have any reason to be embarrassed. Giga is derived from the Greek word ‘gigas’, meaning giants and was likely pronounced the same way we say “giant”. It is also how a lot of people, especially those that have worked with computers for a while, use this prefix. More info on the Wikipedia page for giga.
These days, the accepted way of saying giga is with a hard G so I’ll stick to this method.