Earlier quoted context omitted.
If a state did it, it would be a straightforward violation of the Contracts Clause. With regard to Treasury bonds and gold certificates it was the government rewriting its own contracts. It was a violation of the contract on every Federal Reserve Note and bank deposit as well. Redemption in gold had been suspended before, but the value had not been, and in the late nineteenth century the government started redeeming…
The Supreme Court disagreed and held the Gold Clause constitutional: https://en.wikipedia.org/wiki/Gold_Clause_Cases
"Justice McReynolds wrote the dissenting opinion. He protested that gold clauses were binding contracts, and that allowing the administration's policies to stand would permanently damage faith in the government to uphold its own contracts and those of private parties. McReynolds distinguished the cases at hand from the Legal Tender Cases, arguing that in the earlier cases the government sought to continue operating until it could meet its obligations, while the Roosevelt administration apparently sought to nullify them."