It is; Firefly's version of the curse "goddamn". It's set in the far-flung future, so they had some fun with lingual evolution, most notably incorporating bits of Chinese. "Gorram" evolved from "goddamn" the same way "zounds!" evolved from "God's wounds!"
A form of it was actually somewhat popular in the 1800s. It was used by Dickens and possibly coined by him.
TIL :)
"My days of not taking you seriously are certainly coming to a middle."
"Preacher, don't the Bible have some pretty specific things to say about killin'?"
"Quite specific. It is, however, somewhat fuzzier on the subject of kneecaps."