Make sure you are using internet explorer for Gmarket
Once you sign in with your account, Click on My Gmarket at the top right hand corner.
Go to gaccount and click on check content (below gaccount status).
(If you have Gstamps, you can exchange these for discount coupons). Make sure to unblock pop up contents. The discount coupons given varies with the number of gstamps you have collected.

There will be 2 options, apply and exchange. Apply is like gambling for discount coupons wherease exchange is just exchanging your gstamps to the standard amount.

To use these discount coupons, go into gmarket and find a product you want to buy. If the coupon is applicable, there will be a coupon box in orange next to the price.You will need to click on this and select the coupon you would like to use. Afterwards, you can order as normal.

Discount coupons have an expiry date when you transfer them from gstamps.