what would cause coolant loss with no external leaks? on 2003 GMC Yukon

I did find my oil level high so I expect I have an internal coolant leak.
does this mean a bad head gasket?

Asked by for the 2003 GMC Yukon
it could if the coolent is milky
I think if I ran the engine the oil would be milky, when I checked the oil the level was very high on the dip stick but not milky. this was after the vehicle sat for a day or so. I also have a loss of coolant, but not external I don't want to run the engine if the oil is contaminated with coolant. Could the problem be intake manifold gasket failure ,or head gasket? What test would determine the problem? Thank you for your expertise Mert