The head gasket forms the seal along the cylinders of the engine block. This gasket comes into contact with the engine oil in the cylinders and probably also the coolant channels that run throught the engine block. I bet you're dealer is just testing the gasket for leaks as a precaution that the excess coolant pressure might have caused. More likely than not, your gasket seal should be fine and dandy.
Hopefully your high coolant level was just due to the fact of some inept mechanic at the corner quickie lube overfilling your coolant reservoir on a cold engine. What would be bad is if they find out that your car's engine is running hotter than normal. Your temperature gauge stays around the middle most of the time I hope?