Funton,
1955 GMC 8 Cylinder pickups used a 30ohm wirewound resistor mounted on the back of the gauge to drop 12v to 6v for the gauge.
The information listed in the parts book is:
7.5120 Unit, Resistance
1516846 10-50 all, 65 all
incl. bracket*Fuel & Temp Gauge*12 Volt
Six cylinder GMC pickups that year were still 6 volt.
5watt 30ohm wirewound