The Arduino can only output 5V so there is no way to directly drive the meter to 9V. If you can cope with the complexity of a transistor circuit, you could use the Arduino to turn the transistor on and off (using a PWM control scheme similar to the current one) and use the transistor to switch a 9V supply to the gauge on and off.
Here is the deal with most basic dc meter movements. They are current operated devices made to several standard current ranges say 0-50ua, 0-1ma, 0-25ma, etc. What turns such a meter movement into a voltmeter is a series resistor (called a 'multiplier' resistor in meter talk) that limits the maximum meter current to coincide with the maximum applied 'measurement voltage to the series resistor/meter circuit. This is not unlike sizing a resistor for a simple led, but instead of having to know the Vf of the led you need to know the 100% current rating of the meter movement. So if you look around you might find the series resistor for the meter and just change it's value such that 5vdc will drive it to full scale rather then the 9vdc it now requires. Reducing the resistor to approx half what it is should do the trick.Lefty