M1 correlates to gold because the more money is present in the system, the more gold can be bought and the higher the gold price will become.
Central bank forex reserves correlate to gold because a central bank tends to have as much gold as forex reserves on their balance sheet.
Federal Reserve Custodial accounts are a group of accounts that contain money from foreign central banks (mostly treasuries and a some MBS's). In other words these are the U.S. debt holdings of Foreign Central Banks around the world. It is a direct measure in our opinion of how foreign central banks view the stability and value of the dollar, and the current monetary policies of the US. It frequently shows changes in major financial trends far ahead of "the crowd's" awareness. A rising custodial trend is accompanied by rising gold prices.
Increasing U.S. external debt (which can be found here) leads to higher gold prices because the amount of gold held by the Federal Reserve should be linear with the U.S. external debt held by foreigners. Technically, to be solvent, the U.S. should be able to sell all of its gold to buy up all external debt. As of 2013, external debt was $5.5 trillion. The U.S. held 8133.5 tonnes (or 260272000 oz’s) of gold. To be solvent, the gold price would need to be $5.5 trillion/260272000 = $21131/ounce. This is 15 times higher than the current gold price.