Let's look at the details in a pair of charts.
Fed's Balance Sheet Summary

Total Balance Sheet: $7.093 Trillion

Treasuries Owned Outright: $4.431 Trillion
Mortgage Backed Securities Owned Outright: $2.025 Trillion
Treasuries Plus Mortgages: $6.456 Trillion
Gold: $11 Billion
Gold Comments 

The gold certificate account reflects the receipts issued to the Reserve Banks by the Treasury against its gold holdings. In return, the Reserve Banks issue an equal value of credits to the general account of the Treasury, computed at the statutory price of $42.22 per troy ounce. Because nearly all of the gold held by the Treasury has been monetized in this fashion, the Federal Reserve Banks' gold certificate account of $11 billion represents the nation's entire official gold stock.

If the Fed valued gold at $2,000 per ounce instead of $42.22 per ounce it would be worth about $0.5 trillion. 

What's in the Gap Between Treasuries + Mortgages and the Total?

The Fed publishes balance sheet details every Thursday in a Federal Reserve Statistical Release, affectionately known as H.4.1.

Note: Fred has the Fed's Balance Sheet Total (Less Eliminations from Consolidation) Wednesday Level at $7,093,161. 

The Fed's H.4.1 shows $7,053,440. In the detail below, I show the main assets on the detail chart below adding the final category of Other to make it balance.