Ammeters typically aren't rated for 6 or 12 volts, it doesn't matter.
Electric fuel gauges typically ARE. "old" says he has no issues running 6 Volt fuel gauges on 12 Volts, and he may be right, although I can't believe it does them any good to operate at double design voltage.
However, when I convert a 6 Volt system to 12 I install an electronic regulator to drop the voltage to the fuel gauge to 6 Volts nominal, actually more like 7 Volts.
There's dozens of "do it yourself" circuits out there to do this with a simple little integrated circuit voltage regulator, or they can be bought ready-made.
Another possible issue... if the system was originally (+) ground and has been converted to (-) ground some fuel gauges are not as accurate as before, after the conversion.
Below is ONE source from them ready-made.