You have to output an "analogWrite(pin, value)" from a pin - the "value" will be the number of your geiger counts per minute (for example). The meter is connected through resistor and a filtering capacitor. The resistor shall be set such it reads full scale with 255 value.
The "100V" analog meter is not usable for you (you are not going to measure voltages up to 100V), you need a plain analog meter, typical currents for full scale reading of such analog meters are from 10uA to 1mA.
Tell us what is your analog meter's current for full scale reading (in uAmps). If you are going to measure it, be careful not to burn the meter with high currents. ALWAYS use a RESISTOR wired in SERIES with your analog meter.
Use for example 10k resistor in series, and increase the voltage such you read full scale (see below). The current for full scale will be (aprox.):
I[A] = V[V] / 10000 [ohm]
for example with 1Volt (and 10k resistor in series with the meter) and full scale reading the full scale current is 100uA. http://arduino.cc/en/Tutorial/PWM
analogWrite(pin, 0) - analog meter shows 0
analogWrite(pin, 255) - analog meter shows full scale
With "map" function you can adjust the analog reading range to your geiger count range, for example :
int geiger_count; // this is the geiger count "per minute" for example, range 0..1000 per minute (that is too high!!!)
int analog_meter_value; // this is the analog meter range (0-0, 255-full scale)
analog_meter_value = map(geiger_count, 0, 1000, 0, 255);