Hi The ICN airport to Seoul City 6015 bus is running in mid and end October 2022. Buying a ticket at the airport either from a machine near the airport terminal exits OR better by speaking to a person at the bus ticketing counter is easy and at 16000 KW pp a bargain. There is an attendant to help you get on the correct bus and assist with luggage. Ours spoke some English and we chatted about family while we waited. an excellent intro to Korea.
The problem for a westerner is retracing the journey back to the airport.
We were told by the reception at Hotel Kukdo (at one of the city stops on the 6015 route) that we could not pre-book bus tickets. Further research indicated that you might be able to book via the bus companies app but I think you then need a Korean mobile phone number and a Korean credit card to do this? I was told also that the Seoul transit card is not accepted by the bus? The bus driver refused us boarding as we tried to pay with a Oz credit card, that we had used frequently in South Korea the previous two weeks. He mainly wanted cash or payment with a Korean credit card. We had no local currency left. A group of Japanese fellow travellers then paid for our two fares. The world is like that sometimes.
Also it should be noted that the early 05:16 6015 bus was full only half way around its city loop and had to stop picking up waiting passengers. There was then another bus following from a different route but was this also full? Our bus then took 75 minutes to reach T1 at ICN despite the early start and going against most of the traffic congestion. My advice is to take a taxi back to the airport and allow at least 90 + minutes and probably even two hours for this journey.