Yesterday I took the new train link between Gimpo International airport (subway line 5) and Incheon International airport. The link is run by a different train company but you can use the same T-money card that you use on the Seoul subway system.
The train ride took about half hour and was largely above ground. Not long after Gimpo it stops at Gyeyang (line 1) so you can join it there if you wish.
The carriages were nice and wide with seats along the length under the windows. After dark it’s quite boring because you can’t look out of the window, but there are tv screens mounted on the walls showing the usual comedy clips and news reports one finds on airport trains.
In daylight passengers are treated to views of paddy fields, traditional farm houses, greenery and the usual grey concrete expanses of modern South Korea. At one point as the train passes over the bridge to Yeongjong-do, the island where the airport is sited, you feel as if you are gliding over sand flats.
I think it’s a nice way to come into Seoul after a long flight and it avoids the traffic jams, but be warned, once you get into the subway system you may have some long walks between platforms if you need to change lines.