How to apply to the airline for the 72 hr transit, and how to apply for G-Visa

Hi, may I know which city you plan to transit? if it is Beijing, you can get 72 hour free transit permit directly at the airport on arrival. But if you transit via other cities, you need to inform the carrier airline when check in so that they can email your information to the airport before your landing.
Transit G is required if your itinerary does not apply to the free transit. This application should be down in advance from Chinese embassy/consulate.