i am flying from seoul to Sri Lanka and have 6 hours layover in Guangzhou Airport.

Well, if you wanna leave the airport, you have to apply for the stay permit at the airport first. Then you are allowed to leave. However, according to your limited stopover, I'm afraid that the time is not enough because you need to go back to the airport 2 hours in advance. Thus, you are suggested to stay at the airport and go shopping at the duty-free shops. Or have some foods and take rest in several fast food restaurants and cafe shops inside terminal building. :)