I would just like to add that the instrument is tied to ground to eliminate any floating negative voltage which is apparently very hazardous.
Reading between the lines, I assume that:
the gauge is powered by the 4-20mA signal loop,
the loop to the gauge is fed from the safe area through an IS Barrier,
the power to the whole instrument loop is 24v from the safe area power supply.
If my assumptions are correct, then your statement:
I would just like to add that the instrument is tied to ground to eliminate any floating negative voltage which is apparently very hazardous.
is the key to the problem.
Have a look at the link provided by Dougy83, this shows good and bad earthing practice.
Having the negative side of the loop connected to earth out at the tank sounds like a very bad idea.
From experience, I would expect the body of the gauge to be electrically connected to earth, somehow bonded to the tank or local earth bar, whatever you have.
But I would expect the electrics inside the gauge to be completely isolated from the case and earth.
JimB