How late is your arrival? Because the AREX might be another alternative. Their timetable is here:
http://www.arex.or.kr/jsp/eng/time/time_table.jsp
At Gimpo you can change to an express subway to the Gangnam area, but I'm not sure what time the last train is from there. You can probably google it if you're interested.

I once arrived at ICN very late (about 01:30 I think) on KE, with the flight being delayed due to a snowstorm. KE had some special KAL limousine buses run for the passengers arriving on my flight, including one to the Gangnam area. I simply got off at the nearest stop to where I wanted to go and took a taxi the remaining distance. Quiet easy and much cheaper than a taxi. The last time I took at taxi to ICN was several years ago and it cost me something like 70,000 KRW and I believe prices have gone up since then. I simply cannot see throwing that kind of money at a taxi when the buses to/from ICN are so convenient.