what are the symptons of head gasket probs? i have creamy gung in my engin?

help, im female, bought a vauxall astra 03 from a car sales garage, dident even check the oil, i noticed about 3 months after buying it, and the guarentee expiring, that creamy colured gung was in the engine, and the capp when i went to look at oil level. now few more months later, the gung is slightly smeared on the dip stick. was i ripped off by a car dealer? and is this a sign of gasket trouble? the water level is ok, and my car does not over heat?