I am assuming you will be at the Incheon International Airport.

Golden Gate Casino is located two minutes from the airport. There is a FREE shuttle bus that runs every fifteen minutes, although it depends on what time of the day your layover is. Here's a screen capture from their website:

Here are the current chips (as of November 2010):

You will need to present your passport for entry into the casino. Ask them for a players card, it's free. Here's the current one (as of November 2010):

If you get anything different I will trade for it.

You must buy into a table game to get chips because they will not sell from the cage/cashier.

Here's their website:
http://www.goldengateparadise.com/kr/main/

There are three casinos in Seoul, but there isn't enough time to get there. The traffic can be very heavy and I would not risk it with your time restraints.

Email me for any detailed info you need. Good Luck!