They can make it illegal to privately hold gold. Wouldn’t be the first time in history.
Good luck trading your gold for goods and services under these conditions.
I don’t think outlawing gold would stop the trading. Drugs are also illegal. Also in a crisis such as the one being discussed the laws and the concept of legality doesn’t mean much.
Btw it is estimated that the last time gold was declared illegal, only about 5% of gold owners gave it up.