If you are a student with Advanced Standing eligibility who wishes to take the option of one semester of Advanced Standing, you can apply for a one-semester leave of absence. If granted, the leave would take place during your sophomore year, in order not to conflict with departmental requirements and independent work. Details and deadlines for submitting the application can be found in the Undergraduate Announcement.
Students often talk about taking a "gap year," regardless of the reason for their time away. That term normally refers to time taken after high-school graduation and before starting college. The only official gap year program at Princeton is the Novogratz Bridge Year Program, a service program for students before their first year at the university.