As it may logically sound, gold certificates were a form of currency that was directly linked to the holding of physical gold. The notes were merely an easier form of carrying gold in a note rather than in coin form. The value of gold at the time the notes were produced was a statutory rate of $20.67 per troy ounce, and the notes were basically made in a direct representation of the actual minted coins rather than any gold that might have been stored in bar form.
Like other issues of currency, gold certificates were made in the larger size from 1863 until the introduction of smaller size notes in 1928, when the gold certificates changed size with the Federal Reserve banknotes with which they concurrently circulated until the end of the gold standard for the U.S. dollar, enacted by President Franklin Roosevelt in 1933. At that point in time, gold, thus the currency representing the coin gold, became illegal to own, and vast amounts of the currency was destroyed.
The law that repealed the notes was changed to lift the restriction on the yellow seal gold notes in 1964, primarily so collectors could exchange and own the notes legally, but the same changes in the law removed any representation of actual gold that the notes had. They were effectively Federal Reserve Notes at that time.
Gold notes are very easily distinguishable from any other note in that often the serial numbers, backs, and treasury seal on the notes were printed in a bright yellow-orange ink that was easy to see from afar. It is likely that all of them have left circulation at this point since it has been more than 80 years since they were last made.