The fact that Trump capitalized the words "Gold Standard" may have piqued the interest of those who believe in sound money principles. Trump has in fact spoken in the past about a return to the Gold Standard, and some of the issues surrounding this are summarized in an October 2018 article: Trump Puts Gold Standard On The Table.
A simple interpretation of Trump's tweet means that the U.S. economy is the envy of the world, the benchmark by which other economies measure themselves.
Nevertheless, Trump's tweet can be viewed as valid in another way, whether this interpretation was intended or not. When the U.S. dollar was de-linked from the value of gold in 1971, the U.S. dollar and its economy became the Gold Standard. The U.S. dollar is the primary reserve currency throughout the world, and therefore almost everything bought or sold in the world has a reference point to the value of the dollar.
The value of the dollar is related to the health of the U.S. economy, and the U.S. economy, absent a "real" gold standard, IS the monetary standard throughout the world.