Take the train. The A’REX will whisk you from Incheon Airport straight to Gimpo Airport in 35 minutes for just 3,750 won (~US$3.50), with trains every 10 minutes or so. Note that you’ll need to take the regular “All Stop Train” (commuter) service; the express goes straight to Seoul Station.
There are also bus services, which can be marginally faster if traffic is in your favor, but run less often and cost about twice the price (7,500 won).
You can take the train from Incheon to Gimpo.
The line is named AREX
It takes 34 min and costs 3550 ₩ (~3 € / $).
Credit:stackoverflow.com‘