I stayed in the Lotte Gimpo airport for two nights and was very happy with it.  This would not be an ideal hotel to stay in if you are visiting Seoul as it is 30-40 minutes away by metro, but if you are flying it is very convenient to the airport. 

Check In

Check in was quick and easy, and check-in personnel spoke fluent English.  Elevator to the rooms is very close and hotel staff can assist with bags if needed.

Room

Very comfortable bed and bathroom.  Room is effectively soundproofed due to proximity of the airport.  Ample power ports available for recharging devices.

Location

This hotel is a clearly marked short walk from the international terminal at Gimpo airport.  It is possible to walk to and from the domestic terminal via an underground walkway.  The hotel is also connected to a shopping mall, which expands the food options somewhat.  There is also access to the Seoul metro via the underground walkway between the hotel and the domestic terminal.  If flying from the domestic terminal, give yourself a little extra time as it is a longer walk (maybe 10 minutes between the mall and the check in counters).