Why is my GMC truck is using a lot of coolant?

Your truck is losing coolant for one or more reasons including; a leak outside the engine, typically the water pump, or heater hose (should be noticable in the engine bay or on the ground), or leaking due to a failed head or head gasket (should be noticable by coolant in your engine oil- eg. white/milky oil, or noticable by thick white smoke/steam at the tail pipe especially for a short time after engine start-up.