Airport limousine buses run directly and take about 30 minutes from Incheon Int’l Airport to Gimpo Airport.
It is a frequently used mode of transportation from the airport and operates every 10 to 30 minutes. You can take Bus No. 6003 or 6014 at ICN Airport to GMP Airport.
You can take AREX trains from Incheon Int'l Airport to Gimpo Airport. Be sure to take a train that stops at Gimpo Airport. More info
You can take a Taxi from Incheon Int’l Airport to Gimpo Airport and it takes about 40 minutes.