"Going gold" is the shortened term for "the gold master build" or the even shorter "GM."
A golden master is sent to replication compaines to make the glass masters by which we get our pressed CDs. I heard from one of our senior fellows at IBM that they think it started with either the term "build goal" or "golden hell." If it was build goal, then term comes from the project completion date, the date when the master build is delivered to manufacturing.
If it comes from "golden hell" then that term comes from the developers who decide what platforms, hardware and other factors to leave out of the software. If you tried to include every permutation in your software, then you'd be regression testing for years.
If memory serves me, there was a PBS special/show that detailed the short history of the rise of computers, from Xerox Park to now. (2000?) I think they discuss the term "gold" in that show, too.