Taxi to the dormitory. You can take a taxi from Gimpo to the dormitory. The taxi stand is just outside of the arrival terminal. KW30,000 should be adequate for the trip by a "regular" (not "deluxe") taxi from GMP to the dormitory. Please show the driver this map that shows the route from GMP to the dormitory, and the close-up of the neighborhood of the dormitory. The dormitory may not appear in the GPS navigator, but the driver should be able to locate it from the map.
Subway to Wangsimni Station. On the other hand, the subway would be the most economical and predictable mode of transportation to move from GMP to Wangsimni Station, although it is slow.
Gimpo Airport (GMP) is located at the western edge of the city of Seoul, which is served by 3 subway lines (Number 5, 9 and A'rex). After getting out of the customs area at the airport, look for the sign to the subway. You will have to use Number 5 (Purple) line, which will take a little more than 1 hour to Wangsimni Station. The subway station is conveniently connected to the arrival area of GMP by the underground corridor.
In fact, there is a faster route, but you have to change the subway line once along the way. You take A'rex toward Seoul Station, and get off at Hongik University Station to change to number 2 (Green) line, which will take you to Wangsimni station. It will take a little less than 1 hour.
You have to walk some distance from A'rex to number 2 line through the underground corridor, which is equipped with escalators and elevators. Given the distance, I would consider the faster route, if I am traveling light.
If you plan to use the subway more than once while in Seoul, buying a "T Money" Card is recommended, which is available at a modest fee in most convenient stores and subway stations. Otherwise, you need to buy a single use card, whose instruction is here. The machine has English instructions.
Get out of the station through Exit 6-1 (NOT Exit 6). You may get out through Exits 4, 5 or 12 also. (Exit 4 has an elevator.) Avoid Exits 6 and 13.
When you arrive in Wangsimni Station, the area will look like this although you will be much closer to the station than the vista point of the photo.
From Wangsimni Station to the dormitory. Click here to continue.