Almost every Korea guide explains the Incheon transfer and then stops. Gimpo gets a sentence, which is odd, because Gimpo is the easy one.
The thing to understand first
Gimpo is inside Seoul. Incheon is on an island outside it. That one difference decides everything else: shorter journeys, lower fares, and a subway ride instead of an airport express.
Your options from the terminal
Subway. Two lines serve the airport directly, which covers most of the city with at most one change. For almost every central destination this is the answer.
AREX all-stop. The airport line's local service also calls at Gimpo, which is useful if your hotel is near one of its stops or you are connecting toward Incheon.
Taxi. Much cheaper than the equivalent from Incheon because the distance is a fraction of it. Worth it with heavy bags, a group, or a late arrival — not otherwise.
Which flights actually use Gimpo
Domestic routes, and short-haul international services including several from Japan and China. If you are adding Seoul to a Tokyo or Osaka trip, there is a reasonable chance you arrive here rather than at Incheon — and that is good news.
The trap: Gimpo and Incheon are not two terminals
What is checked here and what is not
The lines serving Gimpo and its position inside the city are stable and checked on the date above. Fares, first and last services, and which airlines use which airport all change — confirm those before building a tight connection.
