It's impossible to use gold as money when it's taxed as a collectible. Does the Constitution implicitly forbid such a tax? I think such a case can be made under Article 1, Section 10:
"Section 10 - Powers prohibited of States
No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts..."
This says that a State may make gold and silver coin a [legal] tender. In effect it grants a power or reserves that power to a State. But if the federal government taxes gold and silver coin transactions, then it interferes with that power. The Constitution certainly doesn't create such a contradiction. It has to be the case that the Constitution never intended for Congress to lay taxes on transactions in gold and silver. In my opinion, the current laws doing this are unconstitutional.