Rather…do you want to use one? True Type Fonts have a great many characters and symbols that go beyond just the routine letters and numbers. Some have more than others. If you want to use them as part of your text in Revit you can use the Windows Character Map application to copy and paste them or make a note of the key combination to “type” them in.
Assuming you are in the process of either typing text or editing text...the “trick” is to use the numeric keypad, the ALT key and type in the number assigned to the symbol. First you enable the numeric keypad, then you press and hold the ALT key and finally type the number (diameter symbol for Century Gothic is 0216). When you let go of the ALT key the symbol appears.
Note, you can’t use the numbers along the top of the keyboard, you have to use the keypad area. Using a notebook computer, you’ll need to type the letters assigned to the NUM lock function.