The airport closer to the city. Tiny but easy to navigate. The walk from arrival to the Metro is a bit of a walk, a solid 15 minutes! About 40 minutes subway to the city. Recommended as always should choose an airport that is close to the city.
I did study Hangul for 2 weeks. So I can read some basic stuff. Studied 2 weeks at least make me not scared of their intimidating look, instead it feels more like a long over due Hangul exam for me. Papago (translate app) rocks. The photo translation and sentence translation works for me for most of the times. Knowing Japanese and Chinese help with a lot of things too. Hotel staff speaks Japanese. A lot of places menu may not have English, but have Chinese / Japanese.
Korean Republic Won, KRW, ₩. A lot of blog posts, sightseeing books recommend to exchange cash at places around Chinese embassy in Myeongdong. But I found Gimpo airport has much better rate than every place around Chinese embassy. If you really need money badly, hotel lobby usually has money exchange vending machine.
I reserved a unlimited Koreainfo data-only SIM for 10 days that costs ₩38,500. Picked up at Gimpo was easy without any problem. About 20Mbps solid speed everywhere I went. Free WiFi are available almost every restaurant and cafe I went.
The metro is built underground, 3-5 levels deep. The time to wait between trains is around 5 to 7 minutes, it feels like forever because Tokyo spoiled me. The transfer between lines are very far, most of the time I spent more than 7 minutes.
The other direction of the line usually is not built back to back, you would have to get up a floor, then get down again. Went to the wrong platform, you have to get out, then come back in to the right gate.
A trip costs around 1300₩.
The charging rule of ticket for transfer is also very different, but no need to worry if you got T-money card. The ticket gate is not very efficient. The display signs for which platform goes to where could be improved.
The audio announcement for arriving at a station could be improved. Although I do have advantage that many station name's pronunciation resembles Chinese pronunciations.
Express trains are available that is not obvious to know. You can get the next one or sometimes in another platform for express train. Express for airport saved 25 minutes for me!
Where to stay
I stayed at 2 hotels. One in Mapo-gu (Gongdeok station), One in Gangnam-gu (Eonju station). For sightseeing and touristy things, Mapo-gu is definitely at better location. Gangnam-gu feels like a lot more to dig into for locals.
For hotels, please see my recommendation: Seoul Hotels. If I visit again, I will choose Gongdeok, Mapo area.
How to Navigate
Google Maps and Apple Maps do not work well here. Use Kakao Map, Naver, Konest. Most of the google results I found for things, they share link in Konest. Naver is most responsive while walking. Kakao map is nice to estimate taxi price.
For metro: Hancao, which you find point A and point B, it shows you the route to get there. But it does not show you the optimal route. Sometimes you can take Express train but they did not show you.