What could be the problem(s) here? (radiator)?

2004 GMC Yukon dash said "Engine coolant hot" and temp gauge goes almost into the red after a while of driving. Next day poured about a gallon of spring water in radiator and looked for a leak under vehicle and no drips (vehicle is running) and temp gauge rised to about 3/4 way to the red while running but no warning light came on. My buddy said could be stuck thermostat and radiator leak but just curious if anyone might know something else before I take it to the shop. Thanks for your advice.