Quoting Adorsey:
Probably a sort of alcohol. A guage needs fluid when mounted to the motor. In your case its not really needed, but I would have the same preference to just fill it up.
Wrong. Gauges are filled with glycerine, not the song. Its used in engine bays to dampin vibrations. In your case its a cheap gauge that wasn't completely filled to begin with, and no it can't be filled easily.