Temperature Gauge/Engine temp help

[Duck.E]

I recently just finished a head gasket job on my 72 chevy c10 and before I did that the temperature gauge was fine. I got everything together and went for a drive and shortly after gauge went into red. I let it cool and took it on highway to see if it bony did it at low speeds, still went up into red. Radiator, water pump, and gauge in cluster are all new. I never had a thermostat before so I put one in and started truck and gauge immediately went to red and truck hadn't been ran in 2 hrs. Can someone tell me how to diagnose the problem, is it gauge, sender, etc. Coolant is full, No leaks, and water pump isn't leaking as well.