The gold standard ended in 1933. FDR made it illegal to own more than 5 oz of gold. Nixon just ended the part of the Bretton Woods program where we could trade gold with other countries for $35/oz. This allowed Gold to fluctuate and allowed for inflation to become a more impactful thing (a tax on savings). It was not nice and very destructive, but it allowed us to buy gold again. The actual gold standard (where paper dollars were tradable for gold) ended in 1933 with FDR and the confiscation of gold (which he gave to the Fed).

This confiscation (eminent domain, and proof of a sovereign dictatorship) of all of American's gold was done because the Fed couldn't increase the money supply (a necessity to get out of the Great Depression) because they had to keep 40% of the money supply in gold reserves. Now of course they had been decreasing the money supply for years (the cause of the Great Depression, totally intentional), so where did the gold go that they had before that they needed an influx of more? That is a great question. I think the answer is, they funded the Nazi rise to power, though I can't find source evidence of that. Regardless, it went somewhere.

They also could have just lowered the reserve requirement, which they did almost immediately after the gold confiscation, but the whole thing was obviously planned to steal all of the gold from America.

The Fed still has that gold today (according to their documents). Of course they "gave it back" to the Treasury, except its still in the Fed vaults (NY Fed). Our government can't even get it back from them because the Fed is a sovereign entity (as an agent of the BIS). We have no jurisdiction over them according to our own laws.

So not LBJ, and not Nixon. It was FDR that ended the gold standard.