Hi Keith,
The gauge is internally quite different from the Amps meter, it has next to the wounding of the gauge itself 2 separate woundings and a big 2k7 resistor. I have an idea that the gauge coil is a far less voltage than the input. I tried bypassing the 2 coils and resistor, this gave me a linear readout from 0 to 12 Volts of the gauge. So I guess that the same circuit used for the Amps meter can also be used for this gauge. Except that I don't need a big resistor to tame all the Amps I think.
Regards,
Edwin