Of course, you can buy the air ticket from Guilin to Hong Kong in China. And there are several direct flights available daily between the two cities. According to my experience, it will be cheaper if taking a domestic flight to Shenzhen first and then get to Hong Kong. From Shenzhen Airport to Hong Kong, u can take an airport shuttle to Luohu Port firstly, and then take the MTR to Hung Hom train station in Hong Kong.