There is a bus that runs regularly (think every 10 minutes) between ICN and GMP. You will need to claim your bags and bring them with you on the bus, they will load them in the belly. The cost is about KRW6,000 or about $5 US. You will need some Korean Money, you pay the driver. The bus is outside on the curb from the exit from Baggage claim. Very nice ride, freeway almost the whole way. Grab a free cart at baggage claim, go through the exit and out to the street. You will see many bus stops, just ask for the bus to Gimpo, and they will direct you. DO NOT take a cab, especially the guys who will approach you in the terminal. They are a rip off.