They can declare a value, i.e., $500 per ounce, and then order it surrendered to the government at that price and/or prohibit anyone from selling it for more than the government set price.
Speculation in some goldbug circles is that that won’t work this time — Americans would shuffle off to Canada or even England to get the market price.
The “confiscation” plan this time, this is also speculation, is the opposite: they’d announce a gaspingly above-market price for gold, everybody sells it to the government, laughing all the way to the bank, then the government announces that the official US gold price is now 10x-100x what they just paid, goes back to the gold standard, pays off debtors in gold, and abra cadabra, eliminates debt. Everybody gets screwed, which is why it can’t be dismissed out of hand.