You should enter your school gymnasium (even if it is in a separate building) as part of your K-12 school Property Use. You don't need to break out the gymnasium separately because it does not provide a more accurate score. However, when entering a school, you will be asked to enter the Gross Floor Area (GFA) of the gymnasium.
In addition, the GFA of your school should include all spaces within the building(s), including classrooms, administrative space, conference rooms, cafeterias, kitchens used by staff, lobbies, gymnasiums, auditoriums, laboratory classrooms, portable classrooms, greenhouses, stairways, atriums, elevator shafts, small landscaping sheds, and storage areas.
Note: the property type of “Fitness Center/Health Club/Gym” is not appropriate for a high school gymnasium. If you decide to enter your gymnasium as a separate Property Use (or even a separate property), then you should still enter it with a property type of “K-12 School.”