One problem related to coolant leaking has been reported for the 2003 GMC Sierra. The most recently reported issues are listed below.
My vehicle began losing coolant. I took the vehicle to a mechanic, who has told me that the cylinder head is cracked and leaking. This is due to faulty casting on the cylinder head. The coolant is leaking into the heads, and causing the oil to lose it's ability to protect the engine from wear. I have seen that there are many people with this issue, and it is a very costly repair.