score:6
Well, since it is near impossible for me to live without stable, high-speed mobile network with unlimited data and tethering, I tried going back to the airport just to purchase the SIM card.
When I returned back to the airport and asked the counter, I didn't disclose that I haven't arrived at the purchase date. It seemed that she must check only the profile page (including a photo and ID) and didn't check out the arrival date.
Because I was so desperate for the SIM card and must avoid any potential flag, I went to Incheon, not Gimpo. So I don't know if it is possible to buy it at Gimpo, but from what I mentioned above, they seemed to not confirm it.
Well, according to this post, it seems that you can purchase it at another counter at Incheon International Airport even after it is over PM 10:00! Also, it seems available at Hongik University Station. I wish I knew it in advance...
If I have any chance to visit them down the road, I will update it again.