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 may use the term "gap year" for a variety of reasons; Princeton uses it to refer to time taken after high-school graduation and before starting college. The only official Princeton-sponsored gap year program is the Novogratz Bridge Year Program, a service program for students before their first year at the university. When students take time away after they've matriculated, we refer to it as a "leave of absence."