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I am about 99.9% sure that there is not an issue cooling the engine.
When I got this truck, I realized the temperature gauge wasn't working. After a couple of days of driving, I took the time to check the connection. Sure enough, somebody unplugged it! Well, I turned on the truck to make sure the gauge worked, and it did. But once I started moving, the temperature creeped up as if the coolant was too hot. When I stopped, it went back to normal temperature. Now, this isn't always consistent with stopping and going. But when the gauge goes up, and I turn off and restart the engine, the gauge goes back to normal temperature. I assume this is why the previous owner had the gauge disconnected. Does it sound like I need a new sensor or gauge or what? Whenever the gauge goes into the red, the "CHECK GAUGES" light flickers then comes on. The truck is a 1990 Chevrolet 1500 W/T v6

Plug the temperature gauge in your ass. That will work.

Replace the sensor, the dash gauge seldom fails. The sensor tells the computer something is wrong and the computer turns the lights on .

Start with the least expensive. Either the thermostat isn't working properly, or the sending unit for the guage is malfunctioning. If they both are ok, I'd look at the water pump. The fins inside can wear and not efficiently circulate the coolant.