Check your thermostat!

It may not be a faulty temp gauge at all - it may be a defective thermostat.
Thermostats corrode. All thermostats corrode eventually. So they need to be replaced every once in a while. Once they are corroded enough they freeze (they are supposed to move).
If they corrode SHUT your engine will overheat
If they corrode OPEN your engine will run cool or take forever to warm up. (The thermostat is supposed to be shut until the coolant warms up, then it opens and lets the coolant circulate through the radiator).
Running cool too long is bad for your engine - for one thing it runs rich for a long time - this results in gasoline condensing into a liquid and running down the sides of your cylinder bore into the crankcase. You can wind up with a LOT of gasoline in your oil and gas is NOT a good lubricant.
I would check your themostat ASAP. When you remove it from a cold bike it should be shut. Boil a pot of water, bend a coathanger to lower the thermostat into the water and let it boil for 10 minutes - then pull it out and it should be open.
You can keep your themostat from corroding as long as possible (and the rest of your cooling system including your very expensive radiator). by changing your coolant once a year with high quality antifreeze and distilled water. Regular tap water has all kinds of contaminants that will ruin your cooling system. Antifreeze has sacrificial compounds to prevent corrosion - that means they get used up over time. Once they get used up your antifreeze no longer prevents corrosion and there goes your radiator, thermostat, water pump etc. etc..