Update:

Hi everyone! I'm back from my trip to Asia! I don't know if I picked a lucky day to fly (Thurs 4/3/14), but I made it to GMP from ICN in 45 minutes after landing!

I did check my luggage but there were only 2 people ahead of me at Immigration and I was waved through 5 minutes later. I went out to purchase a bus ticket to Gimpo. As I was walking to the bus, the bus was driving off. As soon as the person there saw my ticket, he immediately called the bus back and I was boarded a minute later. There was also no traffic and I arrived at the Domestic Terminal about 30 minutes later!