If by a "glass top" you mean that the elements are covered by a smooth sheet of glass such as the Corning Top, most Poskim hold that it cannot be Kashered and must be replaced. However, if you merely mean that the enamel coating on the surface under the grates is glass but the food and pots do not come into direct contact with it, it is irrelevant. Ideally the grates should be replaced or blow-torched, but strictly speaking as long as they were thoroughly cleaned with a caustic cleaner it is sufficient.