If the gauge is electrical, you can mount it inside. If it is mechanical, you can mount it inside with a line full of diesel running to it, or install an isolator under the hood that keeps the diesel fuel under the hood and allows a less volitile liquid to be conveyed to the gauge in your cab. I have read that some people mount their gauges on the cowl, outside of the cab. I don't see a problem mounting it inside the cab. FWIW i have a westach electrical gauge and it works great.
my $. 02
-Jason