Get a FelPro gasket and be done with it. I THINK FelPro makes gaskets for GM but not sure so don't take a chance.
Bad radiator caps allow air in the system: Sure they do, but it shouldn't cause the anti freeze to jellify like DexCool does. DUH? Over the years, how many problems has Prestone caused any engine? ZERO, period, end of story and I rest my case.