After reading the intro to NEC 518, I think it depends on the area.
The NEC handbook has a table extracted from NFPA 101. For a gym with equipment, occupancy is calculated based on 50 SF per person. For a gym without equipment, its based on 15 SF per person.
So if you have a room with equipment and its over 5000 SF, that means your occupancy would be over 100 people, and it would be an assembly. If you have a room and no equipment, 1500 SF would be the limit, and anything larger would be an assembly occupancy.