Location: Grand Sumorum Jeju
Map: https://goo.gl/maps/GvmEgN6k1ribtWLa8, https://naver.me/x4bFe5er
Important:
As of September 2021, anyone eligible for a visa-free entry to Korea are required to get K-ETA approval, which is done electronically and should be done before transit. Their website (https://www.k-eta.go.kr) states it can take up to 72 hours to process. There is a minor fee of 10,000 KRW (~$7).
If you are flying from abroad and transiting at Incheon (Seoul) airport, it may be cheaper to buy two separate round-trip tickets: one for your city to Incheon, and another for Gimpo (Seoul) airport to Jeju. You can then take the airport metro between Incheon and Gimpo, which takes approximately 40 minutes and costs around 2 euros. Reservations are not required.