A completely wild guess at 5:40 in the morning...Fort Knox is known for its gold.
This promo aired in what, 1997? '98? So pre-Millionaire? I'd guess with the 6-figure winners like Thom (who, in 1998 was still TV's all-time winner), Kit Salisbury, Wilbur Hicks, etc., this was the one big-money game show on GSN's lineup. So perhaps that's where GSN considered all the "gold" to be, if you would.
I'm probably way off, but it's the only thing I could think of...at 5:40 in the morning.