2002 Pontiac Grand Prix Head Gasket?

Bought a used Pontiac from a an ad, installed new manifold gaskets because of water in the oil. I also installed a radiator, new thermostat, new sensor, new pig tail. Gauge says engine is running hot, temp would go all the way up then down, then up and stop. I connected my ODB code reader, ODB reads same temp and fluctuates with dash gauge. I have had to add water since replacing the radiator, (2 gallons lost during road tests), bled the system till the bleeder looked like it was pissing, coolant has escaped from a new radiator cap twice during this (seems like a lot of pressure), overflow tank has a little excitement going on also, all cylinders reading 40 psi off minimum spec. Heat gun at thermostat housing reads 208, lower hose at engine reads 180. ODB and gauge read 250-253.