At the time there were two currencies being circulated in the USA. The first was what is known as lawful money which were dollars backed by gold or silver and issued by the Treasury department. The other were the Federal Reserve notes which is what we have today. When FDR seized all the gold, the people were required to bring all their gold coins AND gold dollar certificates to their local federal reserve bank and the money was replaced with federal reserve dollars. There were some exceptions but failure to comply could result in a $10K fine and jail time.
Those were desperate times and people are more than willing to give up their rights and liberties when they are scared and this is apparently what happened at that time. FDR pulled off one of the biggest thefts of the USA's history because people's wealth in the form of physical gold was taken and replaced by fiat dollars issued by a private institution. it was the beginning of giving vast control of the economy to the private banking system via the Federal Reserve.
Oddly enough, they did not do this to silver until the 1960s when they stopped using silver in coinage in 1964 and then invalidated all the silver certificates in 1968. By silver certificate I mean what is pictured below. Compare that to the wording on the FRN dollar in your wallet today.
So I don't think the government will take gold again mainly because there isn't enough of it out there in private individual's hands to make much of a difference. In fact, most people alive today have no idea about this history. They think the Federal Reserve notes are the way that it has always been. BTW, Federal Reserve notes are legal tender but not lawful money. I won't get into that distinction here.
Compare this to the note that you have now. There were gold certificates too, but these were all taken by the government and I believe it might still be illegal to hold them. (could be wrong on that one)
