With the regulator pressure adjusting screw backed fully off there should be no more than one or two psi reading on the gauge if he car has been shut off for a day or two. With the engine shut off and pressure bled off at one of the injector hoses (use caution around gasoline) the gauge should go to 0 or 1psi. If it won’t go down the gauge has likely been damaged by over pressure due to the regulator screw being set too tight. Remember the gauge only show 0 to 30 psi however the fuel pump can deliver as much as 60psi.