if it is a "warning" gauge, (the one you can set a flashing red light when it goes above a certain value) then twist the knob labelled "warning" very very slightly, just enough for it to register.
photo is just for reference.
i have 3 greddy warning gauges (h2o temp, oil temp, and oil pressure, and some times the h20 temp will stick full pegged when i shut the car off. i just barely twist the warning knob next time the car is running and it springs back to the correct reading.