Coins are not money until it's monetized -- until the Treasury says they're money. They weren't legal to spend. It was simply a bright gold round disc. They were by order of the Treasury in 1937, melted down.
The irony of ironies is, in order to make this coin totally legal and totally monetized, the buyer will have to give, in addition to millions of dollars it costs to buy it, $20 -- a $20 bill -- to go back to the Treasury.