Depending on where you stay in Gatokae, a free airport shuttle might be a round trip to and from the airport, at certain hours of the day, or it could be just a one-way shuttle to the airport.
How often will the shuttle run?
It depends on the hotel you choose to stay in Gatokae. Some hotels have shuttles that run at regular intervals day and night; others require advance reservations for custom departures and pick-ups. It's important to check the hotel's policy on Expedia before you book.