No, it's supposed to be used just like others, it just has a prefix at its 'base' unit. Milligram and Megagram would indeed be technically right, but no one uses it (edit: in the case of megagram, thanks daveguy).

I suspect this is because, sort of like length, there's many different names for essentailly different orders of magnitude and you'd first go to the largest (a tonne, 1000 kg) before adding on prefixes. I suspect this many-names factor is in turn because they're such historical things that humans have measured and quantified since prehistory, because distance and mass are the first 'quantities' we begin to encounter.

So instead of 793 gigagrams, I'd say 793 thousand tonnes, or 793 kilotonnes.