how to get transit G type?

You should apply for a transit G type at the embassy of PRC. If you are in South Korea now, you should apply for it at the CVASC. You cannot get it when you arrive at the airport; you have to apply in advance.

Actually, if you stop in PRC for less than 24 hours, and after your transit, you will fly to a third country or region, instead of South Korea, you can use the 24 hour direct transit, so you don’t need to apply for a transit G type.