depending on the age of the truck , if this BDI is a curtis the guage might not be self contained, it may be connected to a controller box.
Simply replacing the guage might not fix it, if it has the separate control box the guage hooks to then the box is probably bad or needs adjusting.
If it is a self contained guage then on the back of the guage there will be 2 small trimpots that are used to set the guage parameters according to truck voltage.

And then of course there is always the possibility that the battery might just be bad and needs to be load tested. Voltage readings alone does not tell the story on battery state.