2005 GMC 2500HD overheating

Lots of things can cause an engine to run "hot" including low coolant, slipping fan built, thermostat that is not opening all the way, old radiator hoses where the intake hose is collapsing, timing off, and lastly a bad head gasket. The last is important as the early LLY engines had a bad head gasket design and they were not reliable. If the head gasket blows it can initially put hot gases into the coolant but eventually it can put coolant into the motor oil and then the engine is toast.

I look at a high temp or similar symptom as an early warning sign and the sooner I find the problem and fix it the less costly it will be in the long run.