Open the radiator cap, wait about 5 minutes, check the gauge to see if it is moving to "hot", and see if the water is circulating through the radiator. You will be able to tell if it is because the water in the radiator will be moving. If the water is not moving you may have a stuck thermostat.
If the water is moving and it is still getting "hot", Id find someone that has a lazer thermometer and see if it is actually getting that hot at the sending unit. If it is not, then you have a problem with the sending unit for your temp gauge.