Gatestones are basically heavy, enchanted rocks. Touching it will assess your viability to travel somewhere (typically associated with a magical permission-based system). If you're not able to travel, the gatestone will give you a mild shock and that is that. If you are on the list, a temporary portal will open, allowing you to walk through and instantly arrive at the corresponding waypoint. (And, I should add, if an unauthorized party attempts to use the portal, they'll be tossed back out. Or instantaneously evaporated. Depends on which you prefer.)