Easy there big fella...
First off, these engines are pretty good about staying cool. Mine has always stayed around the N in NORMAL unless something bad happens, like a water pump going out. When you turn the key on does the needle move any? When the key is off the needle will sit down all the way to the left. If the needle moves any at all it means the gauge itself is probably fine.
If it does move, indicating the gauge itself is working, you need to check the sensor (also known as a thermometer) by blocking any air passing through the radiator. You can do this with cardboard or anything that will stop the air from being pulled through by the fan. Once you get the air blocked, let the truck run and watch the gauge. It may take a while for everything to heat up enough to move the gauge so be patient. I would seriously give it at least fifteen minutes to warm up fully.
If it still doesn't move the needle on the gauge we'll discuss changing the gauge and, if that doesn't do it, changing the sensor.