it will read a little high but it should be close enough to let you know when you are getting close to needing gas.
It might be a problem for the tolerance of the gauge. assuming a 12 volt system to 5 volts, 6 volt goes to 14.4 volts.
but real voltage would be 14.5 max takes it up to 17 volts. that might hurt the gauge.
honestly it is pretty cheap to get a 12 volt to 5 volt vr to wire in. I would do that as a separate line to protect the gauge.
or I suppose there might be a shop to rewire it to 6 volts or 12 volts.