Low on anti freeze, thermostat needs to be replaced, radiator fan not working, blend door is not switching over from A.C to heat, this is just a few things but a little bit more information on what is going on would be helpful to diagnosis better.
Also in front of the engine where the grille is you may have a bad inter cooler. Best thing would be to put a piece of cardboard behind the the grille so the the engine doesn't blow cold air into your cat
Search GMC Sierra 1500 Questions
GMC Sierra 1500 Experts