Firefly wasn’t a late-night broadcast, so there’s next to no bad language, at least, not bad language that you might recognise. “Gorram!” (an approximation of “God damn!”) is about as cuss-y as it gets in English, but there are harsher terms used – in Chinese.
The explanation is that when everyone left Earth-that-was and the Alliance was formed, there were two major cultures that were at the forefront of everything in terms of power, that was America and China (remember the Alliance flag? It’s an amalgamation of the flags of China and the United States of America), as a result, people generally speak both English and Chinese to some extent, and the show gets around the watershed rule for bad language by translating the occasional phrase into the latter.
You’ll often hear a character ask, “Dong ma?” (meaning “Understand?”), or affectionately calling someone “mei-mei” (which means “little sister”), but you’ll also hear them exclaim “Wuh de ma!” (roughly “Mother of God”) or “gos se” (“crap”) or, rather hilariously, “Da-shiang bao-tza shr duh lah doo-tze” (“The explosive diarrhea of an elephant”) – oh yes, Mal actually says that during the seventh episode!
Anyway, the letter G is more about language than anything else, best represented by the word “gorram” as in Firefly was gorram good and it sucks it was cancelled!