In other words, since these coins are, and technically remain, legal tender of the United States, then they are redeemable for their face value. A $20 gold piece is, as currency, worth 20 dollars, just like a 1979 penny is worth a penny even though the copper content is worth more.
You could make a case that if they are sold for more than the value assigned that is a captial gain, but until sold they are only "worth" face value.
I can see the confiscation ticket, ($27,000.00, in coin). If forced to hand it back, a check...for $27,000.00