Here's the real info: no issue for Nepali passports if the transit is less than 24 hours. If transit is going to be more than 24 hours, the airlines won't even let you board the plane at JFK if you don't have Chinese visa. For US passport, no such issues. They will put you in a hotel in Guangzhou if your transit is longer than 24 hours, and the airlines will drop you at the hotel and pickup again next day.