This expression refers to someone or something that is dependably trustworthy. The Phrase Finder reports that the simile originated with banknotes that included promises of redemption in gold or silver. The U.S. government can print money today that has no such precious metal backing, and that money is no longer “as good as gold.” The simile was first recorded in Thomas Hood’s Lost Heir in 1845, “Sitting as good as gold in the gutter.” Perhaps there is irony about the lack of intrinsic value of money printed today and the reference to the gutter in that statement.