[Professional Engineer]

Gigawatts is correct. The pronunciation with the “G” versus the “J” sound changes fashion every few decades.

[Erasmus]

I pronounce it like “Jigawatts” for, I guess, two reasons:

1. The root “Giga” is the same as for “Giant,” “Gigantic,” and “Gynormous.” (OK, I was kidding on that last one.)

2. It gives heart palpitations to the occasional oh-so-racially-sensitive soul.