Easiest way to travel between ICN/GMP is actually by AREX (Airport Rail Express).

AREX is comprised of 2 different types of trains:
1) Express (Non-stop all the way to Seoul Station)
2) Ordinary (Stops at all stations) <----------- This is what you want to take.

GMP is located in the half way between ICN and Seoul Station.

ICN-GMP Travel Time : 45 Minutes

Please allocate yourself 1 hour and 30 minutes for comfortable travel as walking distance from ICN baggage claims to ICN airport station is pretty a long walk and so is walking distance from GMP airport station to GMP terminal.