Well... at least to me, one of those is a large amount of a mass that I have some understanding of.

The other is lalaland non-sense.

I'd have even preferred "Billion grams" over gigagram. Although a gram is really hard to intuit about, since it's basically just too small. I can probably reliably tell you if an object I pick up weighs 1kg or 2kg, I probably can't reliably tell you if it weighs 1g or 2g.

That's a great alternative to kiloton.

At least it wasn't 1 megakilogram